As far as I know (I may be wrong), substances seem to have a definite color because they reflect/transmit all the light rays of the given color, and absorb all the lights rays of the remaining colors (of course, we are talking about the visible region only). That means, that if a substance is red in color, it absorbs all the visible region light except red, and reflects red light.
When we say light is absorbed, it means that the electrons in the molecules of the substance get excited, right? Then, since energy levels are quantized, only certain wavelengths get absorbed. An apple, for example; it always appears to be red, no matter much light shines on it. So, that means for how much ever light I shine on it, all wavelengths except those corresponding to red get absorbed, and the electrons are continuously excited. How is that taking place without any deexcitation?
Assuming that if any deexcitation takes place, the energy is released as photons only. If so, all other wavelengths should also be emitted, and the apple should appear white.
How can we definitively say, that the energy gaps are equally spaced? If deexcitation does not take place, all substances, after shining light on them for a sufficient amount of time, should eventually become conductors, but I don't think that happens. Also, if the light source is switched off, the electrons should deexcite, and the apple should appear cyan in color (complementary color of red). However that does not happen. Why?
Best Answer
Under normal circumstances, what you are seeing is the steady state condition where the rate of absorption is equal to the rate at which energy is conducted away or radiated away again, so the material doesn't heat up. As I understand, unless a material fluoresces, the de-excitation happens in the infrared.
Also, you should realize that just because a photon being absorbed means that an electron went from energy $E_1$ to energy $E_{10}$, there is no reason to believe that in de-excitation, the electron will go back from $E_{10}$ to $E_1$ again in one jump. The energy levels of a solid are very complicated (band structure) and has very many finely spaced energy levels that are almost like a continuum. Where there are gaps in the band structure, the material is transparent.
Here is an example of the band structure of a random material (density of states of gold in the middle). I believe colour has more to do with the conduction bands (the upper squiggles) and the relative number of available states.