If I understand this question right, it needs answers in two parts:
[i] Effect of light on color of a crystal
[ii] Changes caused in a crystal when light falls on it
[i]Now answer as regards to the first part:
The color of any compound depends on how the atoms and or molecules absorb light. Normally white light is considered to have all wavelengths (colors) of light in it. If we pass a white light through a colored compound some of the light is absorbed (we don't see the color which is absorbed, but we see the rest of the light) as it is reflected off the surface. This gives rise to the idea of "Complementary Colors". If a compound absorbs light of a certain color, the compound appears to be of the complimentary color. Pls find attached a file containing a table of colors and their compliments
[ii]Answer to second part:
Two separate processes that are going on in crystal.
[a]The first step is called nucleation. This describes the formation of a microscopic chunk of crystal and is still not all that well understood. The second step is the actual growth of the crystal into something macroscopic that you can see.For the most part, light does not have much of an effect on these two processes.
[b] On the other hand, crystal growth does depend on many other things ( like surfaces or impurities tend to nucleate crystals) and light can have an affect on these things.
For example, ice crystals will form on the surface of silver iodide crystals if you illuminate the surface of the silver iodide with white light but will not form if you illuminate the surface with red light.
[c]There are also some hints that the polarization of light from an infrared laser can affect the structure of the crystal also, at least in crystal made from a particular molecule. This would be a direct interaction between the light and the molecules which form the crystal. [d]In some cases, light seems to speed up the crystal nucleation rate from a few days to a few seconds.
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