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If a photon hits a proton, would it have a color? What color would it be?

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5  
I don't think that kind of color is a good quantum number. –  Alfred Centauri Oct 4 '12 at 22:11
    
a single photon (Fock state) doesn't have that quality of color... –  natan Oct 6 '12 at 5:24

4 Answers 4

up vote 8 down vote accepted

Blue.

The proton is way smaller than a wavelength of visible light. But blue light has a shorter wavelength than any other visible color, red light is longer wavelength, blue is shorter, other colors in the middle somewhere.

White light is a mixture of all the colors of light, all the wavelengths in the visible range. If you illuminate the proton with white light, almost all the white light will just go past the proton, not reflect back, because the proton is so small. But of the small amount of light that does reflect back, a higher fraction of it will be blue light and a lower fraction of it will be red light. So the reflection would appear blue.

This is pretty much the same effect that makes the sky blue. Tiny particles in the sky don't reflect much of the sun's light going past them, but of the small amount they do reflect, more of it is blue than any other color.

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So all atoms would be blue? –  Cole Johnson Oct 5 '12 at 6:36
    
@ColeJohnson No - atoms have many internal electron configurations, and the energy differences between these determine which wavelengths are absorbed/scattered. Individual atoms can have all sorts of color, since they are not limited to a single characteristic resonance. –  Chris White Oct 5 '12 at 7:17

See yourself. This is a photo:

enter image description here

From here.

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3  
That's a proton beam, not a proton. –  David Z Oct 4 '12 at 20:42
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The light from a proton beam is not produced by photons scattering off the protons (which would have to be the case if protons were to have a color). It comes from the interaction of the protons with their environment. (Otherwise e.g. every material in the world should be the same color as electrons.) –  David Z Oct 4 '12 at 20:58
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(1) Even if the beam is in a vacuum, there are electromagnetic fields and such that the protons can interact with (2) No, because electrons don't have a color. The colors of real materials come from electron energy level transitions, and the energy levels are a property of composite systems, not of individual particles. –  David Z Oct 4 '12 at 21:08
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@dongle26 No. The point is that a proton doesn't even really have a color. Protons are too small to reflect light of specific frequencies the way molecules or crystals do. –  David Z Oct 5 '12 at 0:43
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@David Zaslavsky well in that case the color is white and the reflected/dispersed light depends on the color of illumination. On the photo the color is green possibly because of the green illumination or the laser. –  Anixx Oct 5 '12 at 2:53

The color of the proton would be the color of the photon you are using. This is assuming the photon scatters off of the proton and goes into your eyes. Our eyes only detect visible light, so we can only see those colors. If the photon you are using does not scatter off of the proton and go back to your eye, then you won't see it at all. Also, I don't believe protons absorb photons, only electrons do. (Correct me if I am wrong physics community). Thus, you can only count on scattering. Then again, visible light's wavelengths, (hundreds of nanometers), are larger than a proton, so it'd be hard to get scatter in the first place. So in the end, I think you won't be able to see it at all if you are using visible light.

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3  
Protons do interact with (i.e. absorb and emit) photons - all charged particles do. –  David Z Oct 4 '12 at 20:43
    
That's right! How silly of me. Thank you. –  QEntanglement Oct 4 '12 at 20:50
    
So is mwengler's answer of "blue" wrong? –  dongle26 Oct 5 '12 at 16:44

If your eye could detect protons, which it doesn't, but if it did, your brain would assign it a color. Not red, not blue, not green or any other combination. It would be proton colored. So the answer to the title question is 'proton colored'.

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re downvote: Sorry, I -was- being a bit sarcastic in response to the title question. But also a bit serious, taking the question at face value. –  Bobbi Bennett Oct 5 '12 at 14:36

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