Why does light bend?

Discussion in 'Physics & Math' started by KneD, Mar 14, 2002.

  1. KneD Le Penseur Registered Senior Member

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    ok, I'll try to be as clear as possible:

    When we 'send' light through a real small puncture or trail, we are able to see that the light is bended to the outside.
    so light that comes in a straight line, will come out of this puncture in wider directions.

    the smaller the hole, the bigger is the bending.
    On school we learn this is a common feature of waves.
    But, this doesn't satisfie me.....what is the real complete explanation???????
     
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  3. Crisp Gone 4ever Registered Senior Member

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    Only bending or also diffracting ?

    Hi KneD,

    Did they really say light just "bends" (changes direction) or did they mention that light diffracts when passing through a small hole. There's a huge difference

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    Bye!

    Crisp
     
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  5. KneD Le Penseur Registered Senior Member

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    bending, not diffracting

    they really meant bending......we are talking about monochromatic light, so everything has the same frequence.

    indeed, with white light, all seperate colours are diffracted, 'cause they are all bended a little bit different.

    we are talking about the same 'bending-effect'.......but how does it come?
     
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  7. Tristan Leave your World Behind Valued Senior Member

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    All i know is that light is bent by gravity. On earth it is barley, if at all, detectable. But near a black Hole, light is bent by the extreme gravitational pushes and pulls. Now the reason for that is because gravity affects anytthing with mass. Light has mass through Photons. But photons are so small that they need a massive amount of gravity to affect them.

    Later
    T

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  8. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

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    Well, this is interesting. I just bought a book on EM and optics and all that, but haven't started reading it. Sorry.
     
  9. Crisp Gone 4ever Registered Senior Member

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    Hi KneD,

    When light passes through another material with a refractive index (dutch: brekingsindex) different from the one in air, its path deviates (this is called refraction). When light passes through a small slit, it is diffracted.

    But I somehow can't help to think that we're talking about something different here. I think there are two possible scenario's to what you might refer to:
    - Either you mean the effect of light's path being changed when it passes through an optical material (refraction). This is usually where things like Snellius' law come in.
    - Or you could mean the effect where light is split in different angles when it passes through a narrow opening of a piece of paper (for example).

    It would be the first time I hear about light's path just being changed when passing through a small piece of paper actually

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    . But be sure to give us some more information if that is indeed what you mean.

    Bye!

    Crisp
     
  10. (Q) Encephaloid Martini Valued Senior Member

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    Photons follow geodesics, the shortest distance between two points, in other words a straight line. Gravity is caused by the warping of space due to the presence of mass. Therefore it is the path through space in which photons travel that is bent. The photons will travel this bent path making it appear as if light bends.

    Hans von Baeyer, in his prize-winning essay, conceives spacetime as an invisible stream flowing ever onward, bending in response to objects in it's path, carrying everything in the universe along its twists and turns, including light.

    If photons travel through a medium such as a glass window, they will refract, but only at the interface between the window and the surrounding air.

    Light does not bend while passing through a small hole, no matter how small the hole may be. It is quite simple to measure the diameter of the sun with a piece of paper that has a small hole. If light bent after travelling through the small hole, you would never be able to get an accurate measurement of the sun.

    In fact, if light could bend travelling through a small hole, cameras would never work.

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  11. ogster Registered Member

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    light can be bent through a massize gravataionl force, like a black hole.but light will bend a very small amout as the paper has mass and thus has a gravtional foce acting around it, but shuch a small foce that u would b hard pusshed to see it in a miroscopic scale.
     
  12. (Q) Encephaloid Martini Valued Senior Member

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    oggie

    Clearly you did not read my post. Either that or you failed to comprehend it.

    Light does NOT bend. Light always travels in a straight line. The gravitational force you speak of is bending or warping the space that light travels through. Therefore the light will follow, in a straight line, the warped path created by the gravitational force.
     
  13. ImaHamster2 Registered Senior Member

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    Or perhaps Oggie understood Q’s post and still chose to view the pin hole problem in Riemann space rather than in a less intuitive General Relativity space. In Riemann space light does “bend”, i.e. follows a non-geodesic path, in the presence of mass. In Riemann space conventional formulas for calculating distance and angles may be used. Also don’t have to explain how there can be multiple distinct geodesic paths between two points.

    This hamster believes that different mathematical models are appropriate for solving or viewing different problems. This hamster guesses that astronomers use Riemann geometry and view light as being bent when modeling a gravitational lens. A diagram showing a gravitational lens effect will likely be in flat space and show light as "bending".
     
  14. KneD Le Penseur Registered Senior Member

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    206
    Getting the discussion on the rail again

    ok, first, Q, I must totally agree with your point, if you say that the bended path of light in the vicinity of a black hole is caused by the bending of space.......(photons don't have mass etc.)

    but......light does bend, I find it hard to explain exactly (my english gives me an extra handicap) so I'll try to translate a part from my physics learning book:

    This feature of light is more or less the evidence that light has a wave-character, next to interference.

    Well, now i think the situation is clear, light bends, and not only by the bending of space.
    Now here's my question again, why does light bend?

    I already thought of the fact that has his own 'radiation-pressure', when the light passes the slit, the outside photons don't have a resistence from the side anymore, and will spread out.
    But with this feature, light will never be able to form such a thing as a laser.

    And my stupid book only gives the explanation that light bends because it is a wave, but I want to know exactly.....HELP!
     
  15. Crisp Gone 4ever Registered Senior Member

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    1,339
    Ah, diffraction it is...

    Hi KneD,

    The effect you refer to is called "diffraction" in most textbooks. The fact that non-lit parts appear in the diffraction pattern (= what you see on the screen) is indeed caused by the wavenature of light. You can very nicely calculate the resulting pattern of dots, but I'll try to give a non-mathematical explanation here:

    - The reasoning begins with the Huygens principle: this states that at every point where light passes an object (eg. the slit) new sources of spherical waves appear.

    - You can see the slit as a (continous) sequence of points that all generate spherical waves towards the screen. Because of the wave-nature of light, these spherical waves will interfere constructively or destructively, depending on the wavelength, distance from the source ...., forming the diffraction pattern.

    - If you work out all the math, which comes down to adding up all spherical wave contributions, you'll see that the diffraction pattern follows a sin(x) / x curve.

    That's about the best I can do without going into Fourier transforms. However, there's a whole theory behind diffraction patterns that works out charmingly well

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    .

    The Huygens' principle is very important in the theory of light; I can't remember reading a quantummechanical justification of it (in terms of photons), and I doubt it is simple to explain. It's not just merely the absorption (and spherical emission) of a stream of photons, but a more fundamental interaction of the matter in which the slit is and the photons itself.

    Bye!

    Crisp
     
    Last edited: Mar 18, 2002
  16. (Q) Encephaloid Martini Valued Senior Member

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    hamster

    Some Reimann tensors predict black holes. Other Riemann tensors simply show various mathematical models whereby curvature of space can exist without the need for a local source of gravity. For example, there exists curvature in spacetime even in the empty space between the Earth and the Sun. Photons travel along those geodesics as well.

    kned

    If you are defining the bending of light as photons traveling on a curved path, independent to their geodesic path, I must disagree. They may interact with the atoms of the paper while passing through the hole, but they will arrive and depart each atom traveling in straight lines.
     
  17. KneD Le Penseur Registered Senior Member

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    we're getting to the point now....


    ok, this is the mind breaking, sleep-killing part........why does this happen.......you could also see this effect when you work with water waves btw, but still, we are just talking about the effect, and not what causes it....

    ok, that's it.........which interaction occurs???

    anyway, it is still killing me

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    and Q, when I talked 'bout the curved path i only meant the curved space in vicinity of a black hole, what makes it look like the light is bended...

    well, have a fine mind-crack 'bout it.
    JOEP! KneD
     
  18. Crisp Gone 4ever Registered Senior Member

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    After consulting some books...

    Hi KneD,

    "ok, this is the mind breaking, sleep-killing part........why does this happen.......you could also see this effect when you work with water waves btw, but still, we are just talking about the effect, and not what causes it.... "

    Some people will argue that talking about the effect (or doing calculations on it) is exactly how you can understand the physics behind it

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    .

    "ok, that's it.........which interaction occurs??? "

    Ok, I looked up some calculations of quantum scattering, and my guess is that it has to do with spin interaction. I am sure that there must be some more detailed description somewhere, but I'll look into that tomorrow.

    Bye!

    Crisp
     
  19. Rick Valued Senior Member

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    3,336
    Crisp,
    isnt Diffraction due to mutual interference of the primary wavelets,or primary sources which pass through the slit.they become secondary source as due to mutual interference,due to which light appears to be bent.
    wait i"ll come up diagram.

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    isnt it true?



    bye!
     
  20. Rick Valued Senior Member

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    sORRY FOR Poor diagrams,but i hope that makes it more clear.


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    bye!
     
  21. Crisp Gone 4ever Registered Senior Member

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    Hi zion,

    I am not sure if the primary sources interfere. Most of the time such diffraction experiments are conducted with lasers, which tend not to interfere until diffracted (= which for me means passing an object and, as you mentioned, "create" secondary sources that spread out spherically, overlap and interfere).

    Bye!

    Crisp
     
  22. Tristan Leave your World Behind Valued Senior Member

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    How could photons not have mass? If they have no mass, they can't exist. Am I right?
     
  23. (Q) Encephaloid Martini Valued Senior Member

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    How could photons not have mass? If they have no mass, they can't exist. Am I right?

    Photons most certainly exist. And they do not have mass. However, photons exhibit properties of mass; ie. energy and momentum. The energy of a photon can be expressed as E=hf, where f is the frequency and h is Plancks constant. Planck has described light as bundles of energy. All of the energy in a photon is in the form of kinetic energy, the energy of motion.

    Normally when we calculate momentum we use p=mV, where p is the momentum, m is the mass, and V is the velocity. Using this formula we find that if light has no mass, momentum must be zero. Therefore we must substitute mass with another variable. In relativity we describe momentum by replacing mass with the total energy (E=mc^2) plus the kinetic energy. We know light travels at c therefore we replace V with c. Now our equation for momentum becomes p=E/c. Finally we replace E with the expression above hf (energy of a photon).

    With this expression we can show how a photon can not only exist but how photons exhibit properties of mass without the need for mass.
     

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