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When the sun is rising/setting, it goes through a phase where the light is bending from the atmosphere. I believe this image will explain much better than I ever could.

enter image description here

Now, if light goes through a prism, it is bent and the different wavelengths bend different amounts, resulting in the rainbow output.

If the light from the sunrise/sunset is bending, then shouldn't we see a rainbow instead of the red tint? At the very least, shouldn't the top of the sun appear more red and the bottom more purple?

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5 Answers 5

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First, it must be said that the picture you provided in your question is extreme. The concept of light bending is true, but the amount that the light bends is nowhere near as large as the picture shows it. The quantification of how much light bends when transferring from one medium to another is called the "index of refraction," and air's index of refraction is very very close to that of a vacuum, so the bending of the light is very small, and the spreading apart of the colors in the light from the bending smaller still. Thus, the prism effect of the atmosphere is too small to notice with our eyes.

If you check the sky after the sun has set (but still providing light), you will see different colors the further up you look from the horizon (at least, I can from my home) in a rainbow-like fashion (although much larger than a rainbow). However, I believe this is due to the Rayleigh scattering of light in the atmosphere rather than any prism effects. (Rayleigh scattering is what makes the sky blue and sunsets red).

Here is a Wikipedia article on atmospheric refraction for more information.

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So with some extremely precise equipment, the top of the sun at sunset would, in fact, be a different color than the bottom? –  David Starkey Jul 16 at 21:46
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From Wikipedia: "Atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality. It affects not only lightrays but all electromagnetic radiation, although in varying degrees (see dispersion in optics). For example in visible light, blue is more affected than red. This may cause astronomical objects to be spread out into a spectrum in high-resolution images." So, yes. –  Joshua Jul 16 at 22:25
    
shouldn't it be more apparent during a lunar eclipse, though? And does this mean that sunsets on Venus are rainbow? –  Supuhstar Jul 17 at 15:55
    
Why should it be more apparent during a lunar eclipse? I don't understand. The clouds on Venus are too thick to even see the sun, so the "sunsets" on Venus aren't even much to speak of, let alone having the sun be rainbow. –  Joshua Jul 17 at 18:02

You don't even need highly specialised equipment to see the colour separation of the sun at low sun angles, a decent zoom lens on a camera will see it, and it's the origin of the "green flash" effect as the sun drops below the horizon. This site offers a good image: http://www.atoptics.co.uk/atoptics/gf15.htm

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+1 for mentioning the green flash. However, even though there are green flashes and even blue flashes, these are very rare and not something visible in normal conditions. Note that the page you linked to explicitly says the image - and the normal explanation given - is if not wrong, at least extremely simplified! –  JohannesD Jul 17 at 11:33
    
A full green flash I agree is rare due to the stability required in the surface air layers to magnify the green rim, but the green rim is a natural occurrence and doesn't require any specific atmospheric conditions other than clear skies. It's not visible to the naked eye (nor should you be staring at the sun with the naked eye) but a camera zoom lens (with an equivalent length of around 200mm) is enough to spot it. –  Dave_J Jul 17 at 11:58
    
Does the green flash appear as the sun finally sinks behind the horizon? I have always been under the impression that it happened as the sun hits the horizon initially, while it is still above the horizon. This latter case makes no sense to me. –  Joshua Jul 17 at 18:04
    
@Joshua The classic green flash occurs as the sun drops finally below the horizon, however small green segments can be seen as the top rim of the sun drops through various inversion layers. See the following video. The strongest flash occurs right at the end, but there are extra glimpses at 40 seconds, and at 1:52 youtube.com/watch?v=LmgrtfXGQmI –  Dave_J Jul 17 at 19:22

There are better answers than this, but I just want to contribute.

As Joshua said,

The quantification of how much light bends when transferring from one medium to another is called the "index of refraction," and air's index of refraction is very very close to that of a vacuum, so the bending of the light is very small, and the spreading apart of the colors in the light from the bending smaller still. Thus, the prism effect of the atmosphere is too small to notice with our eyes.

Where I live, the sunset is usually rainbow colored. (Notably, it is near the sea. It could be mist, but I don't know.) The sun and the horizon appears redder, and as you look upwards, the sky turns green and then blue. (For Googlers:) The reason the sun looks red is just the same reason as the sky looks blue. The sky is blue since blue light scatters more as it hits air and dust particles. The blue light, which came from the sun, appears most from the sky since it is full of dust. The sunset looks red because the red light doesn't scatter as much as other frequencies and it has more atmosphere to go through, thus more chances for the blue light to scatter away (and let the red light reach you).

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The fact that you see the sun as red means that the shorter wavelengths (green, blue, purple, etc.) were significantly attenuated as the sun rays traversed the atmosphere, due to having undergone scattering. It seems to me that while some rainbow effect indeed theoretically takes place, the extent to which it happens is relatively small. Adding this to the fact that the shorter wavelengths are attenuated, it seems to me that to discern the rainbow effect on the rising/setting sun, an optical apparatus with a higher dynamic range than the range offered by the human eye is required.

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While it seems generally correct, this answer has a very speculative tone. Would be better to be more assertive (backing up with solid physical reasoning as appropriate) and/or add references as necessary. –  Kyle Jul 16 at 22:52

If we incident a monochromatic light (assume red light) on a glass layer starting from thinner to gradually thicker and thicker layers of glass, partial reflection increases to $ 16\% $ and returns to zero-a cycle that repeats itself again and again. If the the layer of glass is just the right thickness, there is no reflection at all! And it is to be noted that the cycle of zero to $16\%$ partial reflection by surfaces repeats more quickly for blue light than for red light. In fact, that's the only difference between a red photon and a blue photon (or a photon of any other color, including radio waves, X-rays, and so on)-the speed at which the cycle repeats with the thickness.

When we shine red and blue light on a film of oil, patterns of red, blue, and violet appear, separated by borders of black. When sunlight, which contains red, yellow, green, and blue light, shines on a mud puddle with oil on it, the areas that strongly reflect each of those colors overlap and produce all kinds of combinations which our eyes see as different colors. As the oil film spreads out and moves over the surface of the water, changing its thickness in various locations, the patterns of color constantly change. (If, on the other hand, you were to look at the same mud puddle at night with one of those sodium streetlights shining on it, you would see only yellowish bands separated by black-because those particular streetlights emit light of only one color.)$_1$

If we incident sun light (polychromatic) on a prism, we are passing light over a large area with varied thickness, which can account for the consequence of rain-bow color formation. Here in the case of the earth's atospheric layers we can assume them a glass slabs of uniform thickness which can't form colors as oil puddle does.$_2$


Credits: $_1$ Richard Feynman-QED, The strange Theory of Light and Matter. $_2$ Reference from reliable sources required.

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protected by Qmechanic Jul 17 at 19:22

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