Light energy, in the form of waves, is absorbed by molecules within an object. This absorption of energy activates a variety of chemical reactions that have the potential to damage paper-based collections. This is termed photochemical deterioration.
Shorter wavelengths of light such as UV light have more energy than longer wavelengths. They bombard an object with more energy in a shorter time and so cause photochemical deterioration to accelerate. This is why UV light is more damaging than visible light, although all light is damaging.
Practically speaking, light causes paper, bookbindings, and many media (such as inks and dyes) to fade, yellow, or discolor. Light exposure accelerates the oxidation of lignin, which causes paper to darken. Light also contributes to the weakening and embrittlement of paper fibers, textiles, and leather. Some types of light (incandescent lamps and direct natural light) also produce significant heat, accelerating the rate at which materials deteriorate.
Materials that are particularly sensitive to light include color photographic prints, early black and white images (especially salted paper and albumen prints), cyanotypes, blueprints, diazo prints, and hand-colored images.
Remember that the effects of light are cumulative and irreversible.