Rainbow

[timojin]

This is non spherical water helping oil make rainbows

Why do beautiful bands of color appear in the tiny oil slicks that form atop puddles on a rainy day? More specifically, why does each band have a different color, and why do the various bands remain distinct?

The following answer comes from Dinesh O. Shah; he is Charles A. Stokes Professor of Chemical Engineering and Anesthesiology at the University of Florida at Gainesville:
"When you see an oil film on the road on a rainy day, it gives rise to bands of beautiful colors for the following reason:

"Small amounts of oil are usually present on the road surface (for instance, lubricating oil from cars, trucks and bicycles). When it rains, drops of oil float on the layer of water that collects on the road because the density of oil is less than that of the water--the same reason that wood floats on water. Commercial oil formulations usually contain a surfactant, an additive that causes the oil drops to spread out into a thin film atop the water. That film is thickest in the center of the patch, or oil slick, and thinnest at the periphery.

"Light reflects upward both from the top of the oil film and from the underlying interface between the oil and the water; the path length (the distance from the reflection to your eye) is slightly different depending on whether the returned light comes from the top or from the bottom of the oil film. If the difference in path length is an integral multiple of the wavelength of the light, rays reflected from the two locations will reinforce each other, a process called constructive interference. If, however, the rays reach your eye out of step, they will cancel each other out due to destructive interference.

Sunlight contains all the colors of the rainbow--the famous ROYGBIV (red, orange, yellow, green, blue, indigo, violet). Each color of light has a different wavelength. Hence, a given disparity in the path length will cause constructive interference of certain colors, whereas other colors will not be observed because of destructive interference. Because the oil film gradually thins from its center to its periphery, different bands of the oil slick produce different colors.

https://www.scientificamerican.com/article/why-do-beautiful-bands-of/

Thank you Mr Google

Very interesting observation and posting. Thank you ..

 

[James R]

Thanks for the pictures , but I thought to revive in my mind the forum on water particle a while ago that not all water particles are the same . and to have a rainbow the particles have to be spherical.

No. As long as light enters the droplet, bounces off the back surface and then refracts back out at you, the exact geometry doesn't matter.

 

[exchemist]

Thanks for the pictures , but I thought to revive in my mind the forum on water particle a while ago that not all water particles are the same . and to have a rainbow the particles have to be spherical.

I hope to God you do NOT revive that thread. I recall you advanced some very silly ideas about clouds, which you clung to in spite of explanations from several of us.

 

[timojin]

No. As long as light enters the droplet, bounces off the back surface and then refracts back out at you, the exact geometry doesn't matter.

You are putting a restriction "droplet, bounces off the back surface and then refracts back out at you" What about between spherical shape and elliptical shape were the elliptical shape is 45 degree off ?

 

[timojin]

I hope to God you do NOT revive that thread. I recall you advanced some very silly ideas about clouds, which you clung to in spite of explanations from several of us.

They were silly to you but I am not satisfied with the explanation.

 

[James R]

You are putting a restriction "droplet, bounces off the back surface and then refracts back out at you" What about between spherical shape and elliptical shape were the elliptical shape is 45 degree off ?

What about them? Why don't you suggest why that would be a problem, if you think it would be?

 

[timojin]

What about them? Why don't you suggest why that would be a problem, if you think it would be?

I suppose for the sake of appearance there is no problem . My point is the incidence of light into the particle is distance A and the exit will be A + a for an elliptical shape and for a spherical incidence and exit is the same . And so the rainbow colors should be different ?

 

[Michael 345]

What about them? Why don't you suggest why that would be a problem, if you think it would be?

Your forgetting square droplets with black outside walls with only a microscopic pinhole for light to go in but not out
If the light is absorbed inside would we see a rainbow if we cut it open?

 

[James R]

You only see the rainbow when the angles are just right to reflect the light back into your eyes. In fact, it's true to say that even two people standing next to each other see different rainbows; they each have their own set of water droplets that is producing their individual rainbow.

 

[superstring01]

You only see the rainbow when the angles are just right to reflect the light back into your eyes. In fact, it's true to say that even two people standing next to each other see different rainbows; they each have their own set of water droplets that is producing their individual rainbow.

While I was driving home --back to St. Petersburg, FL-- after Hurricane Irma, I saw a rainbow. I felt like Jesus was telling me something.

 

[Dinosaur]

You can see a rainbow when it is not raining on you.

Those who think otherwise should experiment with a lawn sprinkler.

 

[Dinosaur]

BTW: Most rainbows are less than a complete circle due to being cut off by the horizon. Small ones can be created by lawn sprinklers.

A circular rainbow is called a glory. In nature they are only seen from high altitudes like being mountaintops or near to a mountain top.

 

[DaveC426913]

You are putting a restriction "droplet, bounces off the back surface and then refracts back out at you" What about between spherical shape and elliptical shape were the elliptical shape is 45 degree off ?

An elliptical droplet still has the geometry of a spherical droplet. There will always be a section of the surface that is at the correct angle - it might just be a little narrower or wider.

And it's not easy to get an elliptical droplet, unless it's falling rapidly, in which case your rainbow might be rather fleeting.

 

[Kittamaru]

You are putting a restriction "droplet, bounces off the back surface and then refracts back out at you" What about between spherical shape and elliptical shape were the elliptical shape is 45 degree off ?

Ultimately, the restriction doesn't matter - water will, in nature, always attempt to form a spherical form due to the force of the water molecules attempting to pull towards one another. Excepting outside interference, there should always be some amount of refraction - the question, I would imagine, becomes one of intensity (is it bright enough to be visible to us)

 

[adianadiadi]

small droplets or even thin films

 

[exchemist]

small droplets or even thin films

Aha, that is an interesting remark, because the mechanism behind the rainbow colours in thin films is different from that in rainbows.

For general readers:

Rainbow colours in thin films are the result of interference, which is more closely related to diffraction than to the refraction one gets in the water droplets responsible for a rainbow.

You get partial reflection from upper and lower surfaces of the film. If the length of the path light travels within the film is close to 1/4 the wavelength of light (or odd multiples of that), you get destructive interference, so no light is reflected. Whereas if the path is close to 1/2 wavelength (or even multiples of 1/4) the interference is constructive and you get reflection. As white light contains a range of wavelengths, a given path length* will eliminate some colours and reflect others.

This is not a refraction phenomenon.

* I say path length rather than film thickness since you get most reflection when the light strikes the film obliquely, and the angle of incidence obviously affects the length of the path taken by the light between upper and lower surfaces. So the colours suppressed and reflected will be different at different angles of incidence.

 

[Seattle]

Since we are talking about rainbows...many people don't realize that there are double rainbows and that actually every rainbow is a double rainbow. You just don't always notice this due to viewing conditions.

The colors are reversed in the second rainbow from the first rainbow. This also contributes to not noticing the second rainbow since the second rainbow starts with purple (hard to see) and as you move away from the primary rainbow things generally begin to fade (due to lighting conditions) and you don't notice the rest. You can test this out with your garden hose as well.

 

[exchemist]

Since we are talking about rainbows...many people don't realize that there are double rainbows and that actually every rainbow is a double rainbow. You just don't always notice this due to viewing conditions.

The colors are reversed in the second rainbow from the first rainbow. This also contributes to not noticing the second rainbow since the second rainbow starts with purple (hard to see) and as you move away from the primary rainbow things generally begin to fade (due to lighting conditions) and you don't notice the rest. You can test this out with your garden hose as well.

Yes. Sometimes you see a third one as well. There are several orders, but usually only the first is bright enough to be noticeable.

 

[DaveC426913]

Since we are talking about rainbows...many people don't realize that there are double rainbows and that actually every rainbow is a double rainbow.

Potentially a double rainbow. There's only a second rainbow if the the mass of droplet-filled air is large enough for the second angle of refraction to reach the observer. This is more rare.