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The simple model of the colour of reflected light from objects (yes, colour perception is a function of the eye/brain) as I understand it is:

Firstly I will write what I understand happens - which may be the source of my misunderstanding.

Consider white light incident on a material.

  1. Photons of particular wavelength can be absorbed by an atom by causing an electron to jump from its "base" state to some higher energy level.

  2. If the remainder of the incident light is reflected or transmitted the colour of the material is whatever the eye/brain perceives to be the colour of white light less the frequencies absorbed.

  3. The material must absorb a range of frequencies otherwise all colours of reflected/transmitted light would appear white until observed through a spectroscope which would indicate individual frequencies missing from the white light spectrum - which would be too little of the whole spectrum to notice.

Which leads to my actual question ....

If an electron has been "excited" by absorbing a photon from the incident light, surely at some moment later in time it will fall to a lower energy level and re-emit the original frequency absorbed? Hence there will be no "missing" frequencies from the reflected / transmitted light and every object will appear to be white? (but of course this does not happen).

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  • $\begingroup$ " a lower energy level and re-emit the original frequency absorbed?" No a good assumption. The chemical reactions in the cones of the eye are very complicated, so the absorbed energy is redistributed differently. Consider what happens when white light is absorbed by a dark surface. The surface doesn't re-emit white light. $\endgroup$ – Bill N Feb 24 '17 at 18:20
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If you are considering a single isolated atom then it's true that the atom has no way of getting rid of the energy from the photon except by emitting another photon. However as soon as the atom is surrounded by other atoms there are various mechanisms for radiationless decay i.e. transferring the energy of the absorbed photon into channels that don't involve reradiating the photon.

In a gas the excited atom or molecule can collide with another atom/molecule and transfer the excitation energy into kinetic energy. This is known as collisional de-excitation (that Wikipedia article is for collisional excitation, but de-excitation is the same process in reverse).

In a solid the energy can be transferred to lattice vibrations, i.e, heat, which is generally known as quenching. In fact in most solids quenching is so efficient that almost no energy is reradiated as photons. Reradiation in fluorescence or phosphorescence is the exception rather than the norm.

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    $\begingroup$ So the atom stays in an excited state for some period of time? Or is the energy absorbed into modes other than electrons going to higher states, eg, vibrational or other motions of the atom that can then be transferred? But then, is the cross section for that higher than electron excitation, and if so why? $\endgroup$ – Bob Bee Feb 24 '17 at 22:06
  • $\begingroup$ @BobBee: you should ask that as a new question. $\endgroup$ – John Rennie Feb 25 '17 at 9:59
  • $\begingroup$ Well, I may, but your answer as to why it doesn't create excited states that then re-radiate is not then as complete as it could be. I thought your answer was very good, was not trying to be picky, and the exact cross sections are not the issue, just wondering how that could happen as you described (which sounds perfectly right to me, and I'm sure has a good explanation) $\endgroup$ – Bob Bee Feb 25 '17 at 22:01
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Firstly the reflection doesn't only happen at the atomic scale, the structure and arrangement of atoms creating the material (such as crystals) can reflect light on it's own, and you can see this in some camouflage animals who are able to change their colors by changing the structure of their skin.

Then comes your question about the atom, well here too I can think of few things that might happen : photoelectric effect can happen so the energy of the photon is used to free up the electron and converted to kinetic energy, and electric current. Or then comes the process you described where the electron re-emits the photon, the photon re-emitted won't actually be at a direction outward from the material, so it will be reabsorbed withing the material beginning the process again, till the energy is simply dumped and converted to vibration of atoms, which is just heat.

This is only the processes I could think of, maybe there is more to it. I hope I was clear.

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This is an interesting question, the most interesting questions are usally what a child will ask. They are not easy to answer. First we will stick to the particle model of light. Imagine we have a stream of white light which is collimated that is the photons are basically tarvelling in the same direction, they then are passed into a clear tube containing say Hydrogen gas, at the other end of the tube you have a diffraction grating or even a prisum that spreads the white light out, the higher frequency photons bend more than the lower frequncy ones. What do you observe? Dark lines which are called absorbtion lines. From a photon flux point of view, the incomming photons say at 13.4eV are virtually travelling in the SAME direction, these photons pump up the electrons to a higher state. But when the electrons drop back the photons they emit will most likley go in any direction. Thus the photon flux is spread out over the surface of a sphere reducing the intensity by a considerable amount compared to other photons which pass through the gas unimpedded. So what you see is a darkened line at the UV freq say. wheas nearby you may see bright areas in the Violet visible range. The key to this is the direction of the photons after they have been absorbed can be in any direction comapred to the incomming direction of the photon flux. It certainly is not trival. If you stimulate the gas using electrical means, so the gas molecules bang into each other the gas will produce emmission lines where the dark bands occur in the absorbtion spectrum. Hope this helps, this is my explanantion that I came up with 50 years ago when I was deeply puzzled by this experiment in the physics lab. This of course gets very complicated becasue it involves statsitical analysis of a many body system, not all atoms will be stimulated.

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  • $\begingroup$ " the most interesting questions are usally(sic) what a child will ask.". Can you please explain the relevance of that statement to your answer? $\endgroup$ – Clive Long Jan 22 at 11:57

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