Crystals grow when molecules come together in an ordered pattern. Usually, the molecules that will form a crystal start out dissolved in a liquid. For example, in the right conditions, salt crystals can grow out of a container of water with lots of salt dissolved in it.
Light can change the way crystals grow in several ways. One is temperature. When things are exposed to sunlight, they often heat up. And, temperature can have a huge effect on how crystals grow. For example, If you heat up a container of water with some salt crystals in it, the crystals may dissolve back into the water and disappear! However, as the temperature rises, water evaporates more quickly. As the water evaporates, the salt dissolved in the water becomes more concentrated, and crystals are more likely to grow. So, in some cases light can first slow down and prevent crystal growth by increasing the temperature, and then increase crystal growth as the water evaporates.
Another way light can affect crystal growth is less common, but really interesting. Some types of molecules undergo chemical reactions when they are exposed to light. These chemical reactions can change the way that the molecules form crystals, and either speed up or slow down crystal growth. There are many scientists researching the ways that light affects crystal growth at this very moment!
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