Eli5 Why does sunlight “bleach” objects, and make their colors more dull and more white?
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Ultraviolet light has a lot of energy and the waves it is made from are the right size to knock pieces off of molecules. Colors are usually made from pigments and dyes which are colorful chemicals. Many of these chemicals can be damaged by the energy from the ultraviolet light. As the damage accrues, the color goes away. You may notice that on old signs the blue is the last to go. That is because blue reflects more of the light closest to ultraviolet so it takes damage more slowly.
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