Though people have been studying crystals for a long time now, there are still some aspects of crystal growth that are mysterious. There are really two separate processes that are going on in crystal growth.
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, as far as I know. On the other hand, crystal growth does depend on many other things (for example, 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 (see
for some pictures).
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. Light also seems to speed up the crystal nucleation rate from a few days to a few seconds.
Light is not expected to affect the process of crystal growth. The only time that it might have to do so is if the growing crystals are light sensitive -- for example, crystals of silver bromide and silver iodide (which are the stuff of photographic film) would decompose to silver metal when exposed to light so if one is growing crystals of these materials, one does it in the dark.
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