This word is used to describe a chemical that can change shape in response to a specific wavelength of light. To our eyes, this change in shape changes the chemical’s color.
The lenses of some eyeglasses are coated with a photochromic chemical. When the person wearing the glasses steps outdoors, ultraviolet light from the sun changes the shape of the chemical on the lenses. The chemical then reflects more light and appears darker, changing eyeglasses to sunglasses!