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I try to understand reflection explained with photons that are interacting with molecules on atomic level.

For this I would like to start with an example and ask the question based on this:

Say I have a white paper (or any other white material) on the table and sunlight shines on it. It seems that almost all visible light is not absorbed and is reflected back into my eyes.

I know following (I guess :-)):

  • Atoms can absorb and emmit photons with quantized energy levels.
  • Molecules can absorb and emmit photons more complex and in different ways.

But in case of the white paper it seems there is no absorbtion of visible light at all.

what happens to the photons (visible spectrum) that arrive at papers surface?

  • Are the photons that I get into my eyes simple absorbtions and reemittings from the molecular structure of the paper?
  • Are they bouncing off like in classical physics? But then what is the explanation on molecular/photon level?

Can someone explain me the interaction between the photon and the molecules in case of a reflection on a solid surface with white color?

I am not asking about direction of light, not asking how the photon knows where to go. (This is well explained already by Feynman's QED.)

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    $\begingroup$ Note that salt crystals are transparent, same as snow crystals or fog droplets or the hairs in a polar bear’s fur. So if you are explicitly not asking about chaotic refraction from a pile of randomly-oriented crystals, then your question is really about the propagation of light in a transparent medium. Note also that visible-light wavelengths are many, many times longer than the atom spacing in a crystal, so you will need some collective model of coherent scattering. $\endgroup$
    – rob
    Jul 2, 2022 at 18:56
  • $\begingroup$ Hi I reformulated my question. I don't mean transparent material. I would like to know how we can explain the photon reflection from a surface, that will not absorb any visual light. In general how are photons reflected. I found tons of explanations regarding absorbtions and transitions on atomic level, but no for reflections. $\endgroup$
    – mcfly soft
    Jul 3, 2022 at 5:56

2 Answers 2

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I realized that I got a downvote, which normally means the answer is wrong, while I think it is true. But the OP has updated the question, so I take this as a chance to also update my answer.

Udate

It seems you expect one simply answer why different things in nature appear white, but the truth is there are different reasons for different things.

For example, clouds are white because of Mie scattering, that is scattering of photons on particles (here small water droplets). There is almost no wavelength-dependence of the scattering efficiency in the visible range, so that all wavelengths in the incoming sunlight are scattered with the same probability, and as a result the cloud is white. But notice, the incoming light has to contain all wavelengths for that. During sunset for example, the cloud base is sometimes red because illuminated by the sun, which is red when close to the horizon during sunset (the reason is that blue light is scattered out of the direct beam due to Rayleigh scattering that prefers to scatter blue light and is responsible for the blue sky during the day).

Another white material is milk, which is a liquid. It is also white because of scattering, this time on particles within the milk. But again, you need an illumination of a light source containing all wavelengths to get a white substance. In a green room, a glass if milk will appear greenish of course...

You received some comments for a transparent material above. The reason is that transparent materials don't do any changes to the color, there is no absorption. Photons of all wavelengths are passing through. Well, actually that is not the entire story. Light is an electromagnetic wave and thus in classical electrodynamics we think of small dipoles that are induced in the material (this was the point in my original answer). The dipoles emit secondary waves that usually cancel out each other inside the material, but at the surface this results in some backscatter. This is the cause for reflection in transparent glass windows (up to 4% is reflected). This doesn't matter as long as the surface is smooth, you just look through the window. But when you make a scratch or take a hammer and smash the window in many, many small pieces, the pieces will become white. The reason is that you now have a lot of small glass pieces with a lot of surfaces, and reflections occur on each single of them, which in total amounts to a reasonable backscattering, and because each wavelength is treated the same, the effective color is white. Another example is ice cubes. Fresh ice cubes with smooth surface are transparent, but crush them or damage the surface and they become white. (Again, only if the incident light contains all wavelengths.)

You explicitly ask for paper. I'm no expert on this, might be again some different reason. Paper is wood and you need to do chemical stuff (bleeching) to make it white...

However, the point is, there is not that single reason, but a number if reasons why different things appear white.

Original answer

In classical physics, light as being an electromagnetic wave is considered to induce small oscillations, i.e. small dipoles, in the material which radiate secondary waves. If the surface is irregular, you get reflection in all directions, meaning if you shine sunlight on it containing all wavelengths, the surface looks white from every direction. This is giving snow its white color, for example (actually, photons even penetrate the snow a bit and are backscattered on each surface within the snow producing what is called diffuse reflection).

I think the related wikipedia article https://en.m.wikipedia.org/wiki/Reflection_(physics) may help your understanding, it has very nice explanations and intuitive plots if you scroll down to "Laws of reflection" and then "mechanism" and "diffuse reflection".

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  • $\begingroup$ I guess you missunderstood the question. I stated : "I am not asking about direction of light, not asking how the photon knows where to go". . I would like to know about the interaction between the photon and the molecules in case of a reflection. Is it absorb and emmit? I don't think so. $\endgroup$
    – mcfly soft
    Jul 3, 2022 at 6:03
  • $\begingroup$ I see you reformulated your question... maybe this post helps you? physics.stackexchange.com/questions/114996/… $\endgroup$ Jul 3, 2022 at 7:10
  • $\begingroup$ Thanks a lot for your help, but still I do not understand exactly. Do you mean that reflection on a solid Surface is just Scattering? Means incoming EM wave will oscillate charges of the materials surface and reemmit with the oscillated frequency of the charges into other direction? If it is scattering, then in case of white material the matter must be able to oscillate with all visible frequencies to shine back as white in sum. Is that true? $\endgroup$
    – mcfly soft
    Jul 4, 2022 at 15:17
  • $\begingroup$ @mcfly soft In principle yes, the electronic shell of the atoms is deformed, and you are right, the according polarizability is material dependent. In the end this is described by the refractive index. You should have a look here en.m.wikipedia.org/wiki/Polarizability and here en.m.wikipedia.org/wiki/Refractive_index (move down to microscopic explanation) $\endgroup$ Jul 4, 2022 at 16:15
  • $\begingroup$ Thanks a lot for explanation, $\endgroup$
    – mcfly soft
    Jul 4, 2022 at 18:22
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For this kind of phenomenon, we use the Fresnel equations, where neither atoms nor photons appear. At the atomic level, not much happens: the interaction of a single photon with a single atom is tiny. We can wave our hands and talk about these tiny interactions adding up coherently (a difficult calculation), or we can just measure an index of refraction and predict the result directly.

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