View Full Version : Speed of Light Through a Medium?


Mike Hawk
10-13-10, 10:01 PM
Velocity is a vector, so it has direction. Speed is the magnitude of velocity, so it doesn't have direction.Speed is the rate of motion, or the rate of change of position. It is expressed as distance moved (d) per unit of time(t). Speed is a scalar quantity with dimensions distance/time. Speed is measured in the same physical units of measurement as velocity, but does not contain an element of direction. Speed is thus the magnitude component of velocity. Velocity contains both the magnitude and direction components.

My question is, since blue light is displaced more than red, through a medium, the blue light has traveled farther distance, so why would we say that blue light travels slower in a medium. Since it traveled farther in the same amount of time?

Wouldnít that indicate that it traveled faster in a medium?

Pete
10-13-10, 10:12 PM
My question is, since blue light is displaced more than red, through a medium, the blue light has traveled farther distance, so why would we say that blue light travels slower in a medium. Since it traveled farther in the same amount of time?

Wouldn’t that indicate that it traveled faster in a medium?

Hi Mike, welcome to sciforums!
It sounds like you're thinking about white light refracting through a prism, right?
http://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Light_dispersion_conceptual_waves.gif/220px-Light_dispersion_conceptual_waves.gif
That's a great question - it shows that you're independently thinking about how things work to get a good understanding, rather than just taking the texts and teachers at face value.

Yes, the blue light is refracted more than the red light, because it travels slower in the medium.
Yes, this means that the blue light travels a greater distance through the prism.
But, your intuition that the blue light takes the same time to traverse the prism as the red light isn't quite right - it actually takes longer for the blue light to pass through.

Do you see why you wouldn't notice this time difference?

Mike Hawk
10-13-10, 10:16 PM
Thank you! No, I'm sorry. I don't see why. How do we know for sure that it takes longer to travel through the prism?

Pete
10-13-10, 10:39 PM
Because of what you said. It goes slower, and travels further. Therefore... it must take longer, right?

Mike Hawk
10-13-10, 10:42 PM
I'm sorry. I just don't get it. If it travels a greater distance in the same amount of time, then it would be faster. So, how do we know it isn't traveling a greater distance in the same amount of time?

Neddy Bate
10-13-10, 11:42 PM
If it travels a greater distance in the same amount of time, then it would be faster. So, how do we know it isn't traveling a greater distance in the same amount of time?


You could send two laser beams straight through an extremely thick plate of the optical medium. Let one laser beam be red, and the other blue. Put a photocell detector on the other side, and measure the transit times for both light beams.

I have a feeling that is not the kind of answer you were looking for, though.

Mike Hawk
10-14-10, 12:28 AM
Huygens died in 1695 and Newton died in 1726. Did they have photosensors and lasers back then?

The only thing that my teacher could say was that Huygens suggested wave behavior explained it better. All that Newton could say was that it was accelerated when it hit the surface because it went into (fits?) Thatís why Huygenís idea was accepted because the wave behavior made more sense.

But donít all other waves, like sound waves, travel faster through a medium?

I just canít understand how we know for sure that it hasnít traveled a greater distance in the same amount of time? Suggesting that it travels faster through a medium. Have we measured the time it takes for each to travel through the prism and measured it with photosensors? Is that how they know?

Neddy Bate
10-14-10, 01:03 AM
Have we measured the time it takes for each to travel through the prism and measured it with photosensors? Is that how they know?

I don't know. That's why I said that my answer probably wasn't the kind of answer that you were looking for.

You raise interesting questions.

Pete
10-14-10, 01:32 AM
I'm sorry. I just don't get it. If it travels a greater distance in the same amount of time, then it would be faster. So, how do we know it isn't traveling a greater distance in the same amount of time?

OK, to answer your question properly means looking for reasons that a medium's index of refraction in Snell's Law (http://en.wikipedia.org/wiki/Snell%27s_law) is considered to be inversely proportional to the speed of light in that medium. Bear with me, because I'm an amateur. Check what I say against more reliable sources, and don't be afraid to call me out if something sounds wrong.

Theory:
There are some good models of light that suggest that a higher index of refraction follows from a slower speed of light in the medium.
Take a look at the Huygens–Fresnel principle (http://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle), which describes wave propogation by considering each point in a wave front as a wave source:
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b7/Refraction_-_Huygens-Fresnel_principle.svg/500px-Refraction_-_Huygens-Fresnel_principle.svg.png
And also check out Fermat's principle (http://en.wikipedia.org/wiki/Fermat%27s_principle), which is another consequence of Huygen's principle which leads to the same conclusion.

Experiment:
The most direct measurement might be with the Fizeau-Foucault apparatus (http://en.wikipedia.org/wiki/Fizeau%E2%80%93Foucault_apparatus) which can measure the speed of light as it bounces off a rotating mirror. By placing a tube of water in the path of the light beam, Fizeau showed that light travels more slowly in water than in air, which helped to disproved Newton's corpuscular theory of light.
I think that interferometry would provide more precise measurements. For example, Fizeau used his eponymous interferometer (http://en.wikipedia.org/wiki/Fizeau_interferometer) to measure the speed of light in moving water:
http://upload.wikimedia.org/wikipedia/commons/thumb/1/1e/Fizeau_interferometer.JPG/300px-Fizeau_interferometer.JPG

Mike Hawk
10-14-10, 02:15 AM
I apologize. I was just giving you the only answer that my teacher gave me.

I believe that light does slow down through a medium. There are lots of experiments to prove it. I even found one that showed that they stopped light completely.

I just donít understand how we know for sure that longer wavelengths travel faster through a medium than shorter wavelengths.

Red 6220-7800
Blue 4920-5770

What if the blue travels this greater distance in the same the amount of time as the red? Then blue would be faster. Am I missing something regarding speed vs. velocity, or distance vs. displacement? Why do we assume because the blue traveled a greater distance, that it is slower. If it traveled farther in the same amount of time, wouldnít this indicate it was faster?

Thank you!

Pete
10-14-10, 02:24 AM
No need to apologise!

I can't answer your question.
My own understanding is simply that the index of refraction has been shown to correlate with the speed of light in the medium, ie that a greater speed difference across the boundary causes more refraction.


What if the blue travels this greater distance in the same the amount of time as the red? Then blue would be faster.
Yes, it would.
But is there any reason to think that it does travel that distance in the same time?


Why do we assume because the blue traveled a greater distance, that it is slower.
We don't say that it is slower because it traveled a greater distance - we say that it must be slower because it is refracted more. More slowdown leads to more refraction.

Pete
10-14-10, 02:27 AM
I imagine that quantum mechanics should predict how fast certain wavelengths of light travel through a medium as well, but that's beyond my ken.

Mike Hawk
10-14-10, 02:29 AM
In optics, Fermat's principle or the principle of least time is the principle that the path taken between two points by a ray of light is the path that can be traversed in the least time.

So, it is all based on this principle, right? But in all the stuff Iíve learned about speed vs. velocity. It states that if something has more displacement in the same amount of time it is faster.

I'm sure there is a simple explanation but I just can't find it.

Thank you!

prometheus
10-14-10, 04:14 AM
The speed of light in a medium is given by the following: v= \frac{c}{n} where n is the refractive index. I imagine you've seen examples like the refractive index of water is 1.33 or something, however (as always) this is a simplification. Refractive index is not just a constant but depends on the wavelength of the light, and therefore the speed of light for different wavelengths in a medium will be different. From a quick look on wikipedia (http://en.wikipedia.org/wiki/Refractive_index) it looks like that n is bigger for blue light than it is for red light, so red light is actually faster than blue light in a medium.

Another way to see this is that, looking at pete's picture in post #2 the red light is deflected less than the blue light, meaning that the change in speed for red light as it passes into the medium is smaller, or that it is closer to c than the blue light.

James R
10-14-10, 04:19 AM
From a quick look on wikipedia it looks like that n is bigger for red light than it is for blue light, so blue light is indeed faster than red light in a medium.

Shouldn't that be the other way around? Blue is refracted more than red. That's in glass, anyway.

prometheus
10-14-10, 04:21 AM
So, it is all based on this principle, right? But in all the stuff Iíve learned about speed vs. velocity. It states that if something has more displacement in the same amount of time it is faster.

Thank you!

Another thing I've just thought of is this: suppose there is a red photon and a blue photon that enter the prism together. You're assuming they also have to exit together, but there's no reason to assume this is so. This is why the polarization of the light after it exits the prism will in general be different to the polarization of the light that entered. For this reason you can use prisms as polarizers.

prometheus
10-14-10, 04:22 AM
Shouldn't that be the other way around? Blue is refracted more than red. That's in glass, anyway.

see edited post. :P I was getting mixed up between wavelength and frequency.

Pete
10-14-10, 04:56 AM
So, it is all based on this principle, right? But in all the stuff Iíve learned about speed vs. velocity. It states that if something has more displacement in the same amount of time it is faster.
Yes, if something has more displacement in the same time, that means faster.


I'm sure there is a simple explanation but I just can't find it.
The time is not the same.
Blue light takes longer to go through the prism than red light.

Mike Hawk
10-14-10, 11:44 AM
Pete-"The time is not the same.
Blue light takes longer to go through the prism than red light."

You said the time is not the same. That’s what I want to know. How do we know this. How do we know the amount of time?

Red 1.33
Blue 1.343

I know that blue light is refracted more and it is displaced more but how does that show that it took a longer amount of time to travel?

If two objects were measured the one that traveled farther in the same amount of time would be going faster. If the blue light covered more distance in the same time then it would be faster. So how do we know what the amount of time is?

Pete
10-14-10, 11:54 AM
We know the blue light is refracted more than the red light.
This means that the blue light is slower than the red light.
This means that the blue light takes a longer time to travel.

Mike Hawk
10-14-10, 11:57 AM
Pete- ďWe know the blue light is refracted more than the red light.
This means that the blue light is slower than the red light.
This means that the blue light takes a longer time to travel.Ē

I know that blue light is refracted more and it is displaced more but how does that show that it took a longer amount of time to travel?

If two objects were measured the one that traveled farther in the same amount of time would be going faster. If the blue light covered more distance in the same time then it would be faster. So how do we know what the amount of time is?


This comes from the Fermat's Principle, right?

Could you explain explain the Fermat's Principle that light follows the path of least time.

Since the speed is constant, the minimum time path is simply the minimum distance path.

How do we know this? Is the speed of light constant in a medium?

Read-Only
10-14-10, 12:01 PM
Pete-"The time is not the same.
Blue light takes longer to go through the prism than red light."

You said the time is not the same. Thatís what I want to know. How do we know this. How do we know the amount of time?

Red 1.33
Blue 1.343

I know that blue light is refracted more and it is displaced more but how does that show that it took a longer amount of time to travel?

If two objects were measured the one that traveled farther in the same amount of time would be going faster. If the blue light covered more distance in the same time then it would be faster. So how do we know what the amount of time is?

Hi, Mike,

I'm a little short of time at the moment but I can clear up one major point for you. We know beyond a doubt that it takes blue light longer because it has been tested time and again. Photo-detectors are set up on the exit side of the medium and are tuned to very precise frequencies. A burst of light is aimed at the medium and timed. The longer wavelengths (lower frequencies) arrive AFTER the higher ones while traveling through the exact same thickness of medium.

So, when the distance traveled is the *same* - the later arrivals *must* have been traveling slower. ;)

More later, perhaps. :)

Pete
10-14-10, 12:17 PM
Mike, do you have any reason to think that blue light takes the same time as red light to pass through the prism?

I know that blue light is refracted more and it is displaced more but how does that show that it took a longer amount of time to travel?
Refracted more means slowed down more. Slower means it takes longer.


If the blue light covered more distance in the same time then it would be faster. So how do we know what the amount of time is?
We know the time because we know the speed.
We know the speed because we know how much it is refracted.

We know how to get speed from refraction by Snell's law. We trust Snell's law because it makes good theoretical sense (see Huygens-Fesnel principle (http://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle) for the classical model) and it matches up with experiments (first done by Fizeau and Foucault (http://en.wikipedia.org/wiki/Fizeau%E2%80%93Foucault_apparatus)).



If that's not enough, then direct measurements of the speed of red and blue light in various media should certainly be doable, and I wouldn't be at all surprised if they've been done, as Read-Only says.

Read-Only
10-14-10, 12:27 PM
Another *very* quick post because I have to leave home in just a moment. :)

Not have they made direct tests, they continue to be made. On every single large telescope lens during manufacture as a routine matter while checking for chromatic aberrations and other tiny flaws that would impair their use.

The physical measurements and calculations are fine indicators of quality - but nothing can replace actual tests for verification. (Remember the lens and reflector in the Hubble Space Telescope?)

Pete
10-14-10, 12:35 PM
I would have expected such tests to be measurements of refraction, rather than directly of speed?

Read-Only
10-14-10, 12:47 PM
I would have expected such tests to be measurements of refraction, rather than directly of speed?

That HAS to be my last post for a few hours. Wife is walking to the car. <grin>

Indeed, the majority of them are, Pete, just not all. And as you've already explained, refraction *does* indicate speed. ;)

Mike Hawk
10-14-10, 12:49 PM
The speed of light is not constant as it moves from medium to medium. When light enters a denser medium, like from air to glass, the speed and wavelength of the light wave decrease while the frequency stays the same. How much light slows down depends on the new medium's index of refraction.

Refraction indicates speed based from Fermat’s Principle, right? That assumes that the speed of light is constant but it is not constant as it travels through a medium. So I don't get it. I don't understand this principal.

Then I found this that says that the original statement of Fermat's principle was, "The actual path between two points taken by a beam of light is the one which is traversed in the least time." Snell's law and the law of reflection follow directly from this statement. It may be reformulated slightly in terms of optical path length as "Light, in going between two points, traverses the route having the smallest optical path length." In its original form however, Fermat's principle is somewhat incomplete and even slightly in error. Its modern form is "A light ray, in going between two points, must traverse as optical path length which is stationary with respect to variations of the path." In this formulation, the paths may be maxima, minima, or saddle points.

I tried to see if there was an experiment that showed that higher energy traveled slower and I did find one that showed that higher energy gamma rays traveled slower than the speed of light. This confuses me even more because the speed is supposed to be constant in a vacuum.


High Energy Gamma Rays Go Slower Than the Speed of Light?

The speed of light is the speed of light, and that’s that. Right? Well, maybe not. Try and figure this out. Astronomers studying radiation coming from a distant galaxy found that the high energy gamma rays arrived a few minutes after the lower-energy photons, even though they were emitted at the same time. If true, this result would overturn Einstein’s theory of relativity, which says that all photons should move at the speed of light. Uh oh Einstein.

The discovery was made using the new MAGIC (Major Atmospheric Gamma-ray ImagingCherenkov) telescope, located on a mountain top on the Canary island of La Palma. Since gamma rays are blocked by the Earth‘s atmosphere, astronomers have figured out a clever trick to see them from the ground. When the gamma rays strike the atmosphere, they release a cascade of particles and radiation. The Cherenkov technique detects this cascade, and then works backwards to calculate the direction and energy level of the gamma rays. With a 17-metre detector, MAGIC is the largest telescope of its type.
The international team of researchers pointed the telescope at Markarian 501, a galaxy 500 million light-years away that contains a blazar – a supermassive black hole that periodically releases bursts of gamma rays. More material is falling into the black hole than it can consume, and so it gets squeezed into jets that fire off from the poles of the black hole at close to the speed of light. What astronomers call a “blazar” is when the jets of a supermassive black holeare pointed directly at the Earth.
Researchers sorted high- and low-energy gamma ray photons coming from the blazar with each flareup. Since all the radiation was emitted at the same time, and the speed of light is the speed of light, you would expect the high-energy photons to arrive at the same time. But nope, the high-energy photons showed up around 4 minutes later.
So what’s happening? Nobody knows, and this could turn into an entirely new field of physics. The researchers are proposing that maybe the radiation is interacting with “quantum foam“. This is a theoretical property of space itself, and predicted by quantum gravity theory – a competitor to string theory.

I really appreciate your help. Thank you!

Pete
10-14-10, 01:22 PM
Refraction indicates speed based from Fermat’s Principle, right?
Sort of.
I think that more fundamentally, it is based on experimental measurements, and on the Huygens-Fresnel principle.


That assumes that the speed of light is constant but it is not constant as it travels through a medium. So I don't get it. I don't understand this principal.

Try this blog entry: Time is of the Essence (http://decartes-einstein.blogspot.com/2006/07/time-is-of-essence.html)
It describes two paths between A and B, that cross mediums at different speeds. The bent path is the fastest:
http://photos1.blogger.com/blogger/5001/3227/1600/Path.0.jpg
Fermat's principle says that the path of a light ray between two points will always be the fastest path, and that's why it is bent (refracted) when it slows down.


I tried to see if there was an experiment that showed that higher energy traveled slower and I did find one that showed that higher energy gamma rays traveled slower than the speed of light. This confuses me even more because the speed is supposed to be constant in a vacuum.
That's not related to this thread, and seems to be bad journalism. There's a brief discussion of it at physicsforums.com (http://www.physicsforums.com/showthread.php?t=189175).

Mike Hawk
10-14-10, 01:48 PM
I’m sorry. This is just really hard for me to understand. I will try to read more about the Fermat’s Principle and Huygens-Fresnel principle but if anyone else has an easy explanation I would really appreciate it.

Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.

However, other waves like sound travels faster in liquids and non-porous solids than it does in air.

So just because it traveled more distance and is bent more how do we know this means it is slower?

If the blue light is slowed down more does it remain slower forever? Did it lose speed or energy?

The high energy article is over my head so don’t worry about it.

Thanks again!

AlexG
10-14-10, 02:03 PM
Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.



As the photons move from electron to electron, they still move at the constant c. We say the light is slowed, but it isn't. It just takes longer through a medium because of the absorbtion and re-emission.


However, other waves like sound travels faster in liquids and non-porous solids than it does in air.



The method of propagation of sound is different than light. Sound wave are compression waves where one molecule bumps another. The denser the medium, the shorter the distance between bumps and the faster the propagation. In a helium atmosphere, sound travels slower than it does through air.

With light, the denser the medium, the more times a photon is absorbed and re-emitted, so the longer it takes a photon to pass through it.


So just because it traveled more distance and is bent more how do we know this means it is slower?

If the blue light is slowed down more does it remain slower forever? Did it lose speed or energy?



The blue light itself isn't slower, it just takes longer to pass through the medium. If it's bent more, it travels through more of the prism, which means there are more absorbtions and re-emissions. The photons themselves still travel at the constant speed c.

Mike Hawk
10-14-10, 02:09 PM
Okay. That makes sense but my teacher should have said that.

So light never loses any energy or speed and technically it always travels at c. It is always a constant even in a medium, right? So technically light never slows down no matter what. They should tell you this right up front, donít you think?

Thank you so much!!!

AlexG
10-14-10, 02:23 PM
So light never loses any energy or speed and technically it always travels at c.

The photons never lose any speed, but they can, and do lose and gain energy. They do so by changing frequency. Higher frequency photons (the blue end of the spectrum) have more energy than lower (red) photons.

Pete
10-14-10, 02:31 PM
Alex, you're confusing things. Mike is struggling to grasp the classical wave model. He doesn't need to worry about quantum theory.

Mike Hawk
10-14-10, 02:37 PM
The photons never lose any speed, but they can, and do lose and gain energy. They do so by changing frequency. Higher frequency photons (the blue end of the spectrum) have more energy than lower (red) photons.

I thought that they already had their frequency in the white light and that the prism just dispersed them. So are you saying that the prism changed their frequency?

RJBeery
10-14-10, 04:56 PM
I'm not an authority on optics and there's a chance that I'm wrong but, moving through the prism does not change the frequency of anything, it simply separates the already existing photons (with their already existing energy levels) from each other. I believe Alex was mentioning that photons can lose and gain energy in other situations.

I think the experiment that would answer your main question would be to take Pete's prism in post #2 and replace it with a sheet made of the same material but of uniform thickness, standing straight up. Now, strike the material with a ray of white light at a perpendicular angle (straight in from the side). The light would experience no refraction (there would be white light coming out the other side), but before that happened the color of light coming out the other side would not be of uniform color...in other words the red would "come out first".

Mike Hawk
10-14-10, 05:26 PM
Thank you but now I'm really confused. So are you saying that light doesn't lose any energy when traveling through a medium? It only loses energy in other situations? Like what?

It says that assuming a sinusoidal wave moving at a fixed wave speed, wavelength is inversely proportional to the frequency that waves with higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths.

It also says that the wavelength is decreased in a medium with higher refractive index. So does this mean that since the wavelength is decreased in a prism that also the frequency is decreased in a higher refractive index? Does light traveling through a prism lose energy? When it says that the wavelength is decreased in a medium. Do they mean it becomes shorter with higher frequency or longer with lower frequency?

I can understand that it may not lose speed because in air (close to a vacuum) all the energies travel at c but does it lose energy? If it does, how does it lose energy?

RJBeery
10-14-10, 05:56 PM
Technically the energy isn't "lost" at all. If it were, the light would change colors as it exited the other side, even when not refracted. Someone already gave a fine explanation on this, and that is that the photons are being "absorbed and re-emitted" by the atoms in the prism which is a process that takes time, while the speed of the photon's travel between those atoms is the same as it is in a vacuum, c.

Think of it like this: a bunch of different colored photons arrive at a party and want to reach the keg in the kitchen. In an empty room, they would all reach the keg at the same time. However, in a crowded room, running into people consumes time (to say hi, excuse me, etc). Blue has more energy so his lateral movements are more pronounced than red. Larger lateral movements increase his chances of bumping into people. Therefore, red reaches the keg first.:cheers:

Mike Hawk
10-14-10, 06:04 PM
Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?


It only loses energy in other situations that I don't know about, right? Could you just give me one example of the other situations?

RJBeery
10-14-10, 06:12 PM
Sure, gravity affects the energy of photons just as you expect it would. A photon leaving a gravity source loses energy which is known as "red-shifting", and a photon headed for a gravity source gains energy which is known as "blue-shifting". The reason for these names make sense if you think about them.

Mike Hawk
10-14-10, 06:15 PM
Great! Thank you! But is this answer correct? I want to print it to show my teacher.

Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

RJBeery
10-14-10, 06:28 PM
Mike, I am a mere amateur. Your teacher, and most on this forum, would know better than me on this subject, but what you have written is how I understand light traveling through a medium**. The reason your teacher may be telling you something different is for pure mathematical analysis only - in other words, it's easier to make the claim that light's speed is less than c through a medium than it is to try calculating ((photon/atomic interactions)/(unit distance traveled))*(length of time delay per interaction)*(distance through medium), etc...in any event, I wouldn't print this out to "throw in your teacher's face" or something, as you are the one being graded.

**that being said, it has been suggested that this explanation does not completely square with observational data! Such is a topic for another day...

AlexG
10-14-10, 06:45 PM
I thought that they already had their frequency in the white light and that the prism just dispersed them. So are you saying that the prism changed their frequency?

"White" light is really a combination of all of the visible spectrum. The prism doesn't change their frequency, it just seperates them by refraction.

Mike Hawk
10-14-10, 06:51 PM
So what I hate my teacher and I do want to throw it in her face. Somebody has to know if this is the correct answer. I just want to make sure I’m right so I can call her on it. Can someone just say if this answer is correct or not?

Answer: Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

This is the final answer, right?



I just thought about what you said about gravity. I understand that the Doppler shift creates red and blue shifting but it says that gravitational lenses do not have chromatic aberration. That gravity bends the light because space-time is warped not because it makes it lose energy or changes the wavelength.

So I was just wondering if it is blue shifted because the path is curved more and when it leaves the gravity source the path is straighter so it is red shifted. Is this right?

Sorry for asking so many questions.

AlexG
10-14-10, 06:52 PM
Great! Thank you! But is this answer correct? I want to print it to show my teacher.

Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

No, not quite. Light certainly can, and does, lose energy traveling through a medium. When you shine a light through a piece of glass, the glass will warm up. This is due to energy from the light which is NOT re-emitted, but which shows up as heat. Light never loses any speed, but the absorbtion and re-emission is never 100 percent efficient. Light viewed through glass will be less bright than when viewed directly. There will always be energy loses which show up as heat.

Entropy always increases.

Mike Hawk
10-14-10, 06:56 PM
Oh I see. So losing this energy in the form of heat doesn't change the frequency at all, right? You explain things good.

Thank you so much!

AlexG
10-14-10, 07:06 PM
Oh I see. So losing this energy in the form of heat doesn't change the frequency at all, right? You explain things good.

Thank you so much!

Yes, because the energy lost comes from photons which don't make it through at all. It doesn't come a little bit from each photon.

Mike Hawk
10-14-10, 07:19 PM
Thank you! My teacher is still explaining it wrong. They should just say that it takes more time to travel through a medium because of the energy exchange and that light never loses speed but it loses energy in the form of heat. That would be a lot less confusing. I donít even think my dumb teacher knows this. They shouldnít even say that it slows down in a medium without explaining this first.

Thanks again!

AlexG
10-14-10, 07:23 PM
What grade are you in? Your teacher may just be trying to keep it simple. OTOH, your teacher may have a degree in education, rather than in the subject he's attempting to teach.

Mike Hawk
10-14-10, 07:33 PM
Iím in the 10th grade. She is my science teacher and she teaches all the different levels of the science classes. So she should know this. She spends most of the time talking about other stuff. She gets annoyed when you ask her a question and then she tries to make you look stupid in front of everyone. So now Iím going to do the same to her.

Thanks again!

AlexG
10-14-10, 07:42 PM
Good luck.

Pete
10-14-10, 08:29 PM
Great! Thank you! But is this answer correct? I want to print it to show my teacher.

Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

No, it's no right. Alex has confused things by adding the concept of a photon, which is really not useful for what you need to learn in your class. In your class, your are learning about optics, about how light behaves on a large scale, not how individual photons interact with atoms. In this class, you are learning how light behaves as a wave, and light waves do slow down in a medium.

Technically, light waves lose speed when they go from air to glass, and gain speed when they go from glass to air.
Light waves don't necessarily lose energy when they slow down (they usually do lose energy over time as they travel through a medium, because no medium is perfectly transparent, but that's a different thing.)

Technically, the behavior of light photons is different, as AlexG and RJBeery point out. But that's something for you to read about in your own time. It's not something that will help you understand the concepts you need in grade 10 science.

What you need to know is this:
The index of refraction of a transparent material (or medium) tells us how much light waves slow down in that medium compared to in a vacuum.
If a light wave crosses a boundary between one medium and another at an angle, it also changes direction as it slows down, like a car pulls to the left if the left front wheel drives into sand, or like a water wave changes direction when entering shallow water. This is called refraction.
Snell's law is an equation that describes how much a light ray is refracted when going from one medium to another: n_1\sin\theta_1 = n_2\sin\theta_2
Air has an index of refraction of close to 1, which means that light barely slows down at all in air.
Glass has an index of refaction of about 1.5, which means that it takes light about 1.5 times longer to go some distance in glass than it would take in air or vacuum.
The index of refraction of a medium is usually different for different wavelengths (colors) of light. Longer wavelengths are slowed down less and refracted less.
Red light has a longer wavelength than blue light, so blue light is refracted more, which is how white light is separated into different colours in prisms and raindrops.

Trooper
10-14-10, 08:40 PM
It sounds correct to me. Someone asked this question before and even rpenner said it never slows down.

How does light regain its acceleration after passing through a medium?


Light never slows down. But at a microscopic scale, light is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum.

Pete
10-14-10, 09:16 PM
Regarding sounds waves, Mike, it is true that higher density mediums mean a higher speed of sound, unlike light.
But refraction still works the same way.
You can use Snell's law to predict how sounds waves will be refracted just like light waves.

I also suspect you're still a bit hung up on the "same time of travel" thing.
Consider the path of red and blue light through a rectangular prism:
http://lttodg.blu.livefilestore.com/y1poTzFhsvE8O20j-RA8EXzfH1xtL2MjV0BdCPizqHxYsUuWqU5IDZFh_Q7M3eWzfaf fj_jWSxcD0Aea7siey25Kh4KSzcFFvQc/RectangularPrism.png?psid=1
Now red light has the longer travel distance. Does that change the way you think about the speed of the light rays?

AlexG
10-14-10, 09:23 PM
In this class, you are learning how light behaves as a wave, and light waves do slow down in a medium.


No, they don't. It's all absorbtion and emission. There is no difference between light waves and photons.

Pete
10-14-10, 09:42 PM
It sounds correct to me. Someone asked this question before and even rpenner said it never slows down.

How does light regain its acceleration after passing through a medium?
It gains speed, not acceleration. And it's not necessarily "regained" - light that originates in (say) water will still go faster if it passes into air.

Short answer - light waves are an electromagnetic disturbance in a medium that is propogated according to the properties of that medium. The speed of light is a feature of the medium rather than a feature of light.


Longer answer:
Think of water waves. When a section of the surface of a pond bobs up and down, it makes the parts of the surface next to it bob up and down as well. Then those parts make the parts next to them bob up and down, and so on. So you have a wave of disturbance propogating through the pond.
The speed of that wave depends on how fast the water itself responds to the disturbance - it doesn't depend on the wave itself.

Sound waves are similar. The speed of sound in air is slow, because air molecules don't interfere with each other that much, so the disturbance (the local compression of the sound wave) doesn't get passed on very quickly.

Sound waves in water are much faster, because the water molecules are more closely associated. In steel it is faster still.

Now think about a sound wave going from air to water to steel. When the air disturbance bounces off the water, it makes a new disturbace in the water molecules. That disturbance is passed on in the water according to how the water molecules interact... so, the sound wave now travels much faster. Not because the nature of the wave has changed, but because of the way it is carried in the medium.

Light waves work the same way. Every medium (including empty space) has properties (permittivity and permeability) that affect how well the electromagnetic disturbances of light waves are propagated.


Photons?
So far, I've only talked about light waves. I don't have much grasp on the quantum nature of things, but my vague, possibily bullshit understanding is that photons don't really slow down in a medium, they are interfered with in a way that is sort of like being absorbed and re-emitted a very short time later. So, when a photon exits a medium into vacuum, it doesn't so much speed up as just carries on in its usual happy way. I don't feel comfortable thinking about photons too much, because it is too intuitively tempting to treat them like tiny marbles, instead of the wave-particle quantum beasties that I'm led to believe they are.

Pete
10-14-10, 09:54 PM
No, they don't. It's all absorbtion and emission. There is no difference between light waves and photons.
I'm not able to authoritatively disagree, but neither do I feel compelled to simply accept your assertion.

I am able to assert that what Mike is going to find most useful in order to understand optics is a sound grasp of the wave model of light.

The simple conceptual model of light waves I described is sufficient to understand the concepts of optics that Mike has to learn, and that while it is good for him to know that there are deeper concepts underneath, that the wave nature of light is something that emerges from more fundamental things, it is not useful to insist that he knows what those fundamental things are right now.

That's what education is about - it's about learning to use the most appropriate model for a purpose, with an understanding of that model's limitations, and knowing where to go or who to ask if it becomes necessary to dig deeper.

Trooper
10-14-10, 09:56 PM
It gains speed, not acceleration.

No. Iím sorry. It does not gain speed because the light itself never slowed down.
It takes additional time, due to the transfer of energy, but the light itself does not slow down.
Alex is correct.

Trooper
10-14-10, 10:06 PM
I'm not able to authoritatively disagree, but neither do I feel compelled to simply accept your assertion.

I am able to assert that what Mike is going to find most useful in order to understand optics is a sound grasp of the wave model of light.

The simple conceptual model of light waves I described is sufficient to understand the concepts of optics that Mike has to learn, and that while it is good for him to know that there are deeper concepts underneath, that the wave nature of light is something that emerges from more fundamental things, it is not useful to insist that he knows what those fundamental things are right now.

That's what education is about - it's about learning to use the most appropriate model for a purpose, with an understanding of that model's limitations, and knowing where to go or who to ask if it becomes necessary to dig deeper.

Then don't tell him that the wave slows down. It doesn't. Why wouldn't an accurate explanation be helpful? It obviously eliminated some of his confusion. There is no such thing as age appropriate physics. It is what it is. IMHO, it should be explained this way in the high school text books. I'm surprised that it's not.

Read-Only
10-14-10, 10:26 PM
Then don't tell him that the wave slows down. It doesn't. Why wouldn't an accurate explanation be helpful? It obviously eliminated some of his confusion. There is no such thing as age appropriate physics. It is what it is. IMHO, it should be explained this way in the high school text books. I'm surprised that it's not.

That's WAY over the top, Trooper. Learning comes in stages - and a kid in high school is neither equipped for NOR needs to know graduate-level physics.

For example, if he asked how a TV worked, is really isn't necessary to go into the deeper details about different types of cameras, bandwidth of the transmitted signal, transmission losses, AFC circuit, voltages used in the chassis, what chemicals the phosphors are composed of, the aquadag coating on the old-style CRT, the purposes of horizontal and vertical sync pulses, blanking interval, the colorburst, etc., etc. That's just plain silly AND a major overload of information.

kevinalm
10-14-10, 10:42 PM
Trooper,

But it is exactly the 'wave' that does slow down. The sum total of the fields of all the photons and matter charges in the medium. Yes, from a QM standpoint the individual photons move at c, but the wave moves at a speed set by the index of refraction.

At the op's level, they're learning some classical em theory as applied to optics. Throwing in quantum mechanics at this point isn't particularly helpful.

AlexG
10-14-10, 10:44 PM
I'm not able to authoritatively disagree, but neither do I feel compelled to simply accept your assertion.



Research it then. Learn the physics.

AlexG
10-14-10, 10:46 PM
That's WAY over the top, Trooper. Learning comes in stages - and a kid in high school is neither equipped for NOR needs to know graduate-level physics.



This certainly isn't graduate level physics. This is basic physics, and should be taught early.

Trooper
10-14-10, 10:49 PM
But it is exactly the 'wave' that does slow down. The sum total of the fields of all the photons and matter charges in the medium. Yes, from a QM standpoint the individual photons move at c, but the wave moves at a speed set by the index of refraction.

At the op's level, they're learning some classical em theory as applied to optics. Throwing in quantum mechanics at this point isn't particularly helpful.


No, they don't. It's all absorbtion and emission. There is no difference between light waves and photons.Alex, does the wave slow down?

Read only,

You’re right. However, this particular kid wanted to know more. When they ask for more, you should give them more. That’s how you should determine their level of understanding. If he’s already thinking about things like this then the information that Alex gave him is completely appropriate.

Sure, gravity affects the energy of photons just as you expect it would. A photon leaving a gravity source loses energy which is known as "red-shifting", and a photon headed for a gravity source gains energy which is known as "blue-shifting". The reason for these names make sense if you think about them.

I just thought about what you said about gravity. I understand that the Doppler shift creates red and blue shifting but it says that gravitational lenses do not have chromatic aberration. That gravity bends the light because space-time is warped not because it makes it lose energy or changes the wavelength.

So I was just wondering if it is blue shifted because the path is curved more and when it leaves the gravity source the path is straighter so it is red shifted. Is this right?

Sorry for asking so many questions.
There’s a good chance that his teacher is dumb. If he’s studying optics then perhaps, he should ask her if rainbows are polarized.

Read-Only
10-14-10, 11:42 PM
Seems to me that this thread may have become just a bit sidetracked. ;)

The teacher is probably competent for teaching at this grade level and what is being taught IS at the proper grade level.

What is different, in this particular instance is that we have a student who is above average and is interested in following this subject into a deeper level than is taught to the average class. Nothing wrong with that - in fact I think it's excellent! And he's already attempting to learn more than is available in that normal classroom setting. His very presence here - and what he's asking - attests to that.


I also need to note that the expressed motive of "showing up the teacher" is not a good one by any means.

So the proper course of action is to tell him what he's reasonably able to understand and for he and his parents to check on the availability of a more advanced class. Lacking that (the probably best) solution, we should be directing him to some college entrance-level books and let him study at his own pace and even encourage him return here when he hits a point that needs clarification. I'm totally in favor of anyone getting the best education they can.

Trooper
10-14-10, 11:54 PM
Seems to me that this thread may have become just a bit sidetracked. ;)

The teacher is probably competent for teaching at this grade level and what is being taught IS at the proper grade level.

What is different, in this particular instance is that we have a student who is above average and is interested in following this subject into a deeper level than is taught to the average class. Nothing wrong with that - in fact I think it's excellent! And he's already attempting to learn more than is available in that normal classroom setting. His very presence here - and what he's asking - attests to that.


I also need to note that the expressed motive of "showing up the teacher" is not a good one by any means.

So the proper course of action is to tell him what he's reasonably able to understand and for he and his parents to check on the availability of a more advanced class. Lacking that (the probably best) solution, we should be directing him to some college entrance-level books and let him study at his own pace and even encourage him return here when he hits a point that needs clarification. I'm totally in favor of anyone getting the best education they can.

That's great but the point that needs clarification is if it actually slows down in a medium. Two other members claim that the wave slows down.

It does not. The apparent speed slows down because it is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum.

Do you agree?

http://www.steelypips.org/principles/2005_04_17_principlearchive.php#111379031611472347

Trooper
10-15-10, 12:10 AM
edit

AlexG
10-15-10, 12:20 AM
Alex, does the wave slow down?



Light does not slow down, and the light wave and photon are just two aspects of the same thing.

Neddy Bate
10-15-10, 12:26 AM
That's great but the point that needs clarification is if it actually slows down in a medium. Two other members claim that the wave slows down.

It does not. It only takes more time because it is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum.



Right, so all wavelengths are actually travelling at speed c between "emission" and "absorption". Thus, the refraction must be caused entirely by the delay between being absorbed and reemitted.

Is there some explanation for the wavelength-dependance of this effect?

Trooper
10-15-10, 01:44 AM
Yes. Dispersion...http://www.free-smiley.info/happy/happy-smileys-emoticons160.gif

http://en.wikipedia.org/wiki/Dispersion_(optics)

This was his original question.

My question is, since blue light is displaced more than red, through a medium, the blue light has traveled farther distance, so why would we say that blue light travels slower in a medium. Since it traveled farther in the same amount of time?
Wouldn't that indicate that it traveled faster in a medium?

Light is treated as a wave in optics, but between atoms the light is traveling at light speed (c = 3◊108 m/s), but after reading about the group and phase velocity. I'm getting a little confused.

Someone who is more qualified needs to answer this.

http://en.wikipedia.org/wiki/Dispersion_(optics)#Group_and_phase_velocity


But here’s a good interactive link for him and you have to admit that it is age appropriate…http://www.free-smiley.info/happy/happy-smileys-emoticons143.gif

http://www.colorado.edu/physics/2000/index.pl

Pete
10-15-10, 06:43 AM
Research it then. Learn the physics.
You too, Alex.

BenTheMan
10-15-10, 09:34 AM
I haven't read this whole thread, but I've looked at the last few comments. Hopefully this cursory reading has given me enough information on the problem. If I'm off-base, let me know, and I'll try to clear it up tonight.

The confusion comes because you're not separating the classical and quantum behaviors of the light. You are allowed to think of the light as a wave, or as a particle, but not both at the same time. So, if you want to think of light as a wave (i.e. optics), then you must forget anything you know about atoms. Classically, there is a medium---there are no atoms.

If you want to think of light as a particle, i.e. the quantum picture, then you can think about atoms. The light travels through vacuum, and encounters atoms when it enters the medium. In the space between atoms, of course, the light is in vacuum. In the material, though, the light has a higher chance of hitting an electron, geting absorbed, then being reemitted, which we interpret (classically) as a change in the speed of light.

Did this clear up any confusion?

BenTheMan
10-15-10, 09:37 AM
Let me reiterate: one is free to use the classical picture or the quantum picture to describe, but not both. The quantum picture should explain the classical picture.

Trooper
10-15-10, 11:09 AM
Nope, nope, not clear to me.
Holy crap! I thought I knew this but I donítÖhttp://www.free-smiley.info/various/36.gif


which we interpret (classically) as a change in the speed of light.

Ben,

Wait a minute, this is basically indicating the same thing. That it is just the apparent speed that slows down because it is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum? One gives the reason why it slows down and seems to correctly state that the light itself isn't slowing down.

That's what Alex said, right?

Are you saying that you can use either one, but just not both simultaneously. That both statement are correct? It slows down is correct, but it doesn't slow down is also correct.

Is there any chance that you would throw in a little bit of the Copenhagen Interpretation for me?

Thanks!

Read-Only
10-15-10, 11:13 AM
Let me reiterate: one is free to use the classical picture or the quantum picture to describe, but not both. The quantum picture should explain the classical picture.

Agreed. :) And allow me to add just another tiny bit to the picture as requested by Trooper in a PM to me.

In any action, such as an energy conversion, there is at least a TINY amount of time required to complete the process - and in this situation there are two conversions taking place: First, the photon strikes an electron and secondly the electron falls back to the ground state and releases an electron. And of course this process is repeated *millions* of times and those times are additive. So th end result is an over-all delay in passing through the medium. And, just as you said, we interpret - classically speaking - that delaying effect as a reduction in speed. And that interpretation is useful and works quite well however you might choose to use it.

As a rough parallel example, consider how we view and use "centrifugal force" which is actually not a force in itself but rather a result of plain old inertia in action.

Moran
10-15-10, 12:32 PM
I guess Read-Only meant to say: "First, the photon strikes an electron and secondly the electron falls back to the ground state and releases a Photon". Not "releases an electron". Right? I fully understood the explanation though.

Neddy Bate
10-15-10, 03:04 PM
That's all well and good, but no one has explained why blue light is slower in a medium than red light.

Is light with a shorter wavelength more likely to become absorbed by an atom? Or does it take longer for an atom to absorb and re-emit shorter wavelengths of light?

Those seem like the only two possibilities, considering that all wavelengths of light travel at c when they are in-between the atoms.

RJBeery
10-15-10, 03:17 PM
Neddy Bate, in what now seems like an inappropriate metaphor to give a 10th grader asking questions, I explained below how I think of the physical (quantum) description of what is going on...

a bunch of different colored photons arrive at a party and want to reach the keg in the kitchen. In an empty room, they would all reach the keg at the same time. However, in a crowded room, running into people consumes time (to say hi, excuse me, etc). Blue has more energy so his lateral movements are more pronounced than red. Larger lateral movements increase his chances of bumping into people. Therefore, red reaches the keg first.:cheers:

Neddy Bate
10-15-10, 03:35 PM
Blue has more energy so his lateral movements are more pronounced than red.

That makes sense. Thanks, RJBeery!

Read-Only
10-15-10, 03:44 PM
I guess Read-Only meant to say: "First, the photon strikes an electron and secondly the electron falls back to the ground state and releases a Photon". Not "releases an electron". Right? I fully understood the explanation though.

Good catch, Moran, that mistake was the result of trying to type too fast. Sorry, folks.

Mike Hawk
10-15-10, 05:37 PM
Here was my teacher's response. People on science forums on the internet are not scientists. They are only people who like to discuss science. I doubt that any of them even have a bachelorís degree in science.

Light can exhibit both wave and particle characteristics but when discussing the speed of light traveling through any material it is always observed as a wave. When light travels across a boundary between two media it changes direction and it speeds up or slows down. It also changes the wavelength into a wave with a larger or a shorter wavelength.

Thanks for nothing!

Dywyddyr
10-15-10, 05:58 PM
Then your teacher is wrong.
Simple, really.
We have a number of actual, practicing, working, published scientists here on Sci.

arfa brane
10-15-10, 06:07 PM
Light can exhibit both wave and particle characteristics but when discussing the speed of light traveling through any material it is always observed as a wave.
Ask your teacher what happens when single photons are transmitted through any medium. Are they observed as waves, and when they cross a boundary does the wave-velocity change?

I can recommend Richard Feynman's book QED, which I think will help you with your understanding quite a bit, and it's aimed at the general public. I imagine a local library will have a copy.

Pete
10-15-10, 06:28 PM
But none were talking with Mike yesterday.

Mike, I think you only wanted to hear what you already agreed with. You chose to ignore advice that agreed with your teacher, just because you wanted her to be wrong.

brucep
10-15-10, 07:25 PM
Mike

Your teacher seems to be a bit of a know-it-all. PHD's post here and the best answer came from PHD Ben The Man. Unfortunately it was probably after you made the mistake of calling out your teacher. The quantum interpretation will recover the classical physics in the classical domain of applicability. That's how it works. Good luck and you might want to reconsider going to a gunfight without any bullets.

Mike Hawk
10-15-10, 09:48 PM
Brucep - BenTheMan has a PhD in physics? Perfect. You just provided me with a few a bullets. Thanks. I’ll show her your remarks on Monday but I have no idea what this means. “The quantum interpretation will recover the classical physics in the classical domain of applicability.” She’s not just a know it all she is really mean. Everyone hates her.

arfa brane - Cool! I will ask her what happens when single photons are transmitted through any medium. Are they observed as waves, and when they cross a boundary does the wave-velocity change?

BenTheMan- If it's viewed as waves the energy contained in one of those waves should depend only on its amplitude and not on the intensity of the light. The frequency should make no difference just like in the photoelectric effect but refraction and dispersion does depend on the frequency.

So if it depends on the frequency isn’t it more reasonable and correct to view it as a particle when thinking about the speed?

Is this just a case where the concept of a wave is useful to understand it but the quantum picture is more accurate?

Is her explanation wrong or not? Yes or no please.

brucep
10-15-10, 11:22 PM
Mike

Domain of applicability just means the domain where a theoretical model applies. Classical physics is physics that isn't quantum in nature. IE mechanics, electromagnetism, optics, special relativity, general relativity.... My guess is you're studying optics? So I'll just say this. The quantum explanation is the empirically correct explanation. The classical theory of optics is still very useful. Snell wrote his law in the mid 1600's a time when everybody thought the speed of light was infinite but could be slower passing through a medium. Experiment seemed to prove that was the case resulting in the development of the refractive index. At that time they didn't even know the speed of light was finite until it was discovered that the constant in Maxwells equations [mid to late 1800's] was actually the speed of light. So light doesn't slow down in a medium it's just described that way if you evaluate the phenomena using the classical theory of optics. No problem, it's all good as they say. The part about quantum mechanics recovering optics at a classical limit just means if you compare the quantum experimental results with the the classical experimental results they essentially have a 1/1 correspondence just interpreted in a different way. Think about this domain thing before you confront your teacher.

brucep
10-15-10, 11:46 PM
Ask your teacher what happens when single photons are transmitted through any medium. Are they observed as waves, and when they cross a boundary does the wave-velocity change?

I can recommend Richard Feynman's book QED, which I think will help you with your understanding quite a bit, and it's aimed at the general public. I imagine a local library will have a copy.

If you go to Prof Edwin Taylors Home site you can download his QED interactive workbook and simulations. Really cool stuffl

arfa brane
10-16-10, 12:03 AM
The frequency should make no difference just like in the photoelectric effect but refraction and dispersion does depend on the frequency.
You have this wrong. The photoelectric effect does depend on the frequency of the incident radiation.


So if it depends on the frequency isn’t it more reasonable and correct to view it as a particle when thinking about the speed?

Is this just a case where the concept of a wave is useful to understand it but the quantum picture is more accurate?

Is her explanation wrong or not? Yes or no please. Her explanation is correct in the classical domain.

BenTheMan
10-16-10, 12:32 AM
The confusion comes because you're not separating the classical and quantum behaviors of the light. You are allowed to think of the light as a wave, or as a particle, but not both at the same time.


re you saying that you can use either one, but just not both simultaneously. That both statement are correct? It slows down is correct, but it doesn't slow down is also correct.

Sorry...was that not clear?


Is there any chance that you would throw in a little bit of the Copenhagen Interpretation for me?

Thanks!

Why? I don't really like the Copenhagen interpretation, and it's not really relevant here.

Mike Hawk
10-16-10, 12:40 AM
arfa brane- Yes. I think it is correct. I said ďif it is viewed as a wave then the frequency should make no difference.Ē Her answer is correct and you are a 100% positive? Do you have a degree?


So light doesn't slow down in a medium it's just described that way if you evaluate the phenomena using the classical theory of optics.
made by brucep


What brucep is saying sounds correct. She is saying that when discussing the speed of light traveling through any material it is always observed as a wave. That doesn't sound correct.


ďIf it's viewed as waves the energy contained in one of those waves should depend only on its amplitude and not on the intensity of the light. The frequency should make no difference just like in the photoelectric effect but refraction and dispersion does depend on the frequency.Ē
made by me

prometheus
10-16-10, 05:49 AM
Here was my teacher's response. People on science forums on the internet are not scientists. They are only people who like to discuss science. I doubt that any of them even have a bachelorís degree in science.

I know for a fact that lot's of people on here are scientists because I've met them, and indeed, I have a masters in theoretical physics and I'm working on my PhD which I'll hopefully be finishing this year. You can tell that to you teacher, and also ask what their degree is in.

Pete
10-16-10, 05:59 AM
The problem is that Mike didn't ask yesterday who knew what they were talking about, and chose the wrong stuff to give to his teacher, for the wrong reasons (i.e. he wanted her to be wrong).

Read-Only
10-16-10, 10:07 AM
The problem is that Mike didn't ask yesterday who knew what they were talking about, and chose the wrong stuff to give to his teacher, for the wrong reasons (i.e. he wanted her to be wrong).

Agreed, which is why I've given up on this thread. I finally realized, much as you just said, that his only motive here is to slam a teacher that he dislikes. Very childish behavior - and nothing to do with actually learning physics. <sad story, this>

brucep
10-16-10, 02:14 PM
The problem is that Mike didn't ask yesterday who knew what they were talking about, and chose the wrong stuff to give to his teacher, for the wrong reasons (i.e. he wanted her to be wrong).

He's a youngster who might learn something through this discourse [probably the hard way]. So what set of answers do you think he would get if he asked everyone whether they knew what they were talking about ? I's say it would be the set of 'I know what I'm talking about'.

arfa brane
10-16-10, 02:18 PM
I said ďif it is viewed as a wave then the frequency should make no difference.Ē Her answer is correct and you are a 100% positive? Do you have a degree?

You don't need to have a degree to understand the photoelectric effect. It was "discovered" by Einstein who showed that light (radiation) appeared to behave like particles, which determination contradicts the wavelike behaviour of light as in Young's double-slit experiment. Anyone can find out about both these people and what they did in the experiments, and so today everyone who knows about them also knows that light behaves like waves, or like particles, depending on the experiment you do.

Nobody (and I mean nobody) can explain this dichotomy without recourse to either a classical or a quantum model, but it can't be classical and quantum simultaneously as others have pointed out.

Trooper
10-16-10, 08:05 PM
Sorry...was that not clear? The Copenhagen, why? I don't really like the Copenhagen interpretation, and it's not really relevant here.

Iím sorry. I wasnít trying to be a smartass, this time. Since, you used Bohrís complementarity principle, that matter exhibits a wave-particle duality. An experiment can show the particle-like properties of matter, or wave-like properties, but not both at the same time. It is closely identified with the Copenhagen interpretation. So, thatís why I asked.


The problem is that Mike didn't ask yesterday who knew what they were talking about, and chose the wrong stuff to give to his teacher, for the wrong reasons (i.e. he wanted her to be wrong).


Agreed, which is why I've given up on this thread. I finally realized, much as you just said, that his only motive here is to slam a teacher that he dislikes. Very childish behavior - and nothing to do with actually learning physics. <sad story, this>


He's a youngster who might learn something through this discourse.

I like Bruce and heís right. I know of another member, who feels that debating and challenging enhances his ability to learn. The kid has already pointed out that his teacher is annoyed by questions. <sad story, this>. Curiosity should be encouraged, not stifled. Arfa brane provided a good explanation for the wave particle duality thatís along this line.


Most of the time, two competing theories canít exist to describe one phenomenon. But in the case of light, one theory is not enough. Instead of throwing out one theory and keeping the other, physicists maintain a wave/particle duality to describe the behavior of light. It is important to understand that this is not an "either/or" situation. Duality means that the characteristics of both waves and particles are present at the same time. The same beam of light will behave as a particle and/or as a wave depending on the experiment. The wave form of light is actually a form of energy that is created by an oscillating charge.

Quantum mechanics is difficult and can be a barrier for many students completing an education in physics. The intelligent students will probably understand the mathematics but may still have trouble grasping the physical principles. The physical meanings start to become incomprehensible. I think itís supposed to be thought of as an extension of classical mechanics.

So, donít ever let anyone make you feel dumb. Sometimes a little history can help.

Since light seemed to travel only in straight lines, people used to think of light as streams of particles. However, it doesnít always travel in a straight line. If the medium changes then the straight line rule no longer applies, refraction is a good example. We know that Newton was a big fan of the particle theory but it had some issues. Particles would collide and rebound. It also didnít offer an explanation for color. The particles would have to be different, but no one knew how. Newton could explain refraction with his theory, but the particle speed had to be increased, when passing from a low density to a higher one.

Huygensís wave theory could explain all of this, but Newton was heavily admired. It wasnít until 1801 that Young had revived the wave theory with the Young-Helmholtz theory of color, which was the famous double slit experiment. This severely hampered Newtonís particle theory and other evidence started turning up in favor of the wave theory, i.e. Rayleigh scattering, the color of the sky. Huygensís idea finally won approval concerning the velocity of light in various mediums. One experiment that someone pointed out earlier was Foucaultís. It supported Huygensís theory. At this point, the wave theory was finally accepted. Thinkng of light as a wave, lead to many other exciting ideas, i.e. the Doppler Effect. However, the wave theory still had problems and couldnít explain everything.

You couldn't just say that light was only a wave because there are different types of waves. Water waves are transverse, but can also be a combination of both, and sound waves are longitudinal waves. Everyone thought that light waves were longitudinal, but neither the wave or particle theory could explain double refraction, unless it was considered only as a transverse wave. However, if light was a wave, what was waving?

Transverse waves can only be conducted through solids. Now, that light wave considered a transverse wave, we needed a semisolid substance, not just a fluid, because a fluid canít hold a sheer force. Thatís why the idea for ether was born. I think it was Thompson who tried to build a mechanical model with rotating fluid because vortex rings can be very stable and offer more resistance. Maxwell liked this idea and begins to add to this model, which lead him to understand how light is connected to electromagnetism. He is able to calculate what the speed for a transverse wave would be in this model and is within 1% of c.

Analogies are needed for understanding, but they never deliver perfect insight, that's why it is said that Maxwell's physical descriptions are the equations themselves. It was difficult for even the most well known intellectuals to grasp. He used mostly Cartesian notations, because he did not like vector notations, and France and Germany were using laplacian notations, so it was difficult to translate. Hermann Von Holtz agreed with his results but even he could not grasp the actual physical conditions of this statement. I think Maxwell indicated that this field was primary and charges and currents secondary, the charges and currents were not physical entities themselves but a consequence of this field. Many resisted the idea, and did not want to give up the concept of ether, just like with Newtonís particle theory.

Even Einstein's General Relativity did not contribute to ether, and a lot of people resisted the theory of relativity, and some never accepted it. There were no concrete payoffs to allow you to understand practical mechanics. I think Michelson and Lorentz never even came around to accepting GR. The simplicity, unity, elegance, is what came into play for those who did.

Like I said earlier, the theory of relativity didnít state that ether didnít exist, but it did remove the need for it to transmit the force of gravity. However, it was still needed to explain how light, as a transverse wave was transmitted across a vacuum, but Einsteinís special relativity killed it.

P.S. Iím sure someone will follow and make a few corrections. Thatís how it works here, but you can tell your teacher that I said sheís stupid for trying to squash your curiosityÖhttp://www.free-smiley.info/various/yes.gif

James R
10-16-10, 10:04 PM
Mike Hawk:

I have skimmed through this thread. I've seen a lot of correct responses, and some confusing ones. I'd like to reply myself, so excuse me if I repeat some of what other people have said before.

In the classical picture of light, light is a wave. The speed of a light wave is fastest in a vacuum and reduces in any other medium. The speed of a wave in a medium is v = c/n, where c is the speed of light in vacuum and n is the refractive index of the medium. The speed of red light is different from the speed of blue light in most mediums due to the properties of the medium. Another way to say this is that the refractive index, n, of a medium depends on the wavelength of the light passing through the medium.

In the quantum picture of light, light consists of small(ish) packets of energy that move through a medium. Mediums, on the small scale, are mostly empty space, with atoms every now and then. When a photon is moving between atoms, it always moves at c - the speed of light in a vacuum. But when it interacts with an atom in the medium, that interaction takes a little time. In effect you can think of the photon being absorbed then re-emitted by the atom, although that's simplifying the picture a bit. That absorption and re-emission process causes a delay in the total time that it takes a photon to travel through the medium, leading to a lower "effective" speed of light that matches the classical formula v=c/n. The dependence of n on the wavelength has to do with the quantum energy level structure of the particular medium.


But donít all other waves, like sound waves, travel faster through a medium?

It depends on the medium and the type of wave. Look at something like waves in water, for example, and you find that the water depth affects the wave speed - waves travel slower in shallow water and faster in deep water.


I believe that light does slow down through a medium. There are lots of experiments to prove it. I even found one that showed that they stopped light completely.

I just donít understand how we know for sure that longer wavelengths travel faster through a medium than shorter wavelengths.

Snell's law of refraction is derived using the assumption that the speed of light changes in a medium. Since we see red light refracting through a prism differently to blue light, in just the way that Snell's law predicts, we know that the assumption behind Snell's law must be correct.


Could you explain explain the Fermat's Principle that light follows the path of least time.

Fermat's principle comes after observing that light travels at different speeds in different mediums. The question is: once you know the speed of light in, say, air and glass, can you predict what path a light ray will take when it goes from air to glass, and vice versa? The answer is: yes. If you assume that the total travel time must be minimised, then you can derive Snell's law of refraction.


Since the speed is constant, the minimum time path is simply the minimum distance path.

Fermat's principle was invented precisely to deal with two (or more) different speeds, for light going from one medium to another.


Then I found this that says that the original statement of Fermat's principle was, "The actual path between two points taken by a beam of light is the one which is traversed in the least time." Snell's law and the law of reflection follow directly from this statement. It may be reformulated slightly in terms of optical path length as "Light, in going between two points, traverses the route having the smallest optical path length." In its original form however, Fermat's principle is somewhat incomplete and even slightly in error. Its modern form is "A light ray, in going between two points, must traverse as optical path length which is stationary with respect to variations of the path." In this formulation, the paths may be maxima, minima, or saddle points.

This is correct. Note, however, that the term "optical path length" is NOT the same as "path length". Shortest time (Fermat) doesn't generally mean shortest distance (length).


I tried to see if there was an experiment that showed that higher energy traveled slower and I did find one that showed that higher energy gamma rays traveled slower than the speed of light. This confuses me even more because the speed is supposed to be constant in a vacuum.

Was that for gamma rays in a medium or in a vacuum? It sounds like maybe you misinterpreted the experiment.


Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.

That's the quantum picture described at the start of this post.


So light never loses any energy or speed and technically it always travels at c. It is always a constant even in a medium, right? So technically light never slows down no matter what. They should tell you this right up front, donít you think?

No. I don't think so. Looking at what happens on a macroscopic level at first makes much more sense than trying to guess what is happening on an invisible, microscopic level. You can get a prism and refract a ray of light through it, measure angles, and just look at it. But you can't see individual photons or atoms. You need a much deeper understanding to grasp what is happening on the microscopic level.


Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

Summary: in a quantum-mechanical picture, the speed of light between atoms in the medium never changes, but the effective speed of the light as it crosses some macroscopic distance in a medium is reduced compared to the vacuum speed. The amount by which it is reduced is exactly given by the usual classical laws of refraction (Snell, etc.)


Here was my teacher's response. People on science forums on the internet are not scientists. They are only people who like to discuss science. I doubt that any of them even have a bachelorís degree in science.

Quite a few of us here have at least a Bachelor's degree in Physics. Some of us even have PhDs in physics and teach in universities.

Your problem in this thread (and in general on sciforums) is that you have no way to tell if the person giving you a particular answer has any qualifications or not. Some people here will give you rubbish answers or just get things wrong. Fortunately, there are enough qualified people here that they will usually point out the errors those other people make quite quickly.


Light can exhibit both wave and particle characteristics but when discussing the speed of light traveling through any material it is always observed as a wave.

That's not quite right. It really depends on what exactly is meant by "observed". What kind of observation are we talking about? Who is making the observation? What exactly are they doing?

Hope this helps.