Ultraviolet radiation.

The sun produces ultraviolet (UV) light. UV light is a portion of the electromagnetic spectrum with wavelengths shorter than those of visible light but longer than those of x-rays. UV light wavelengths—shorter than the shortest wavelength of visible light, violet—are too short to be seen by the human eye (ultraviolet literally means “beyond violet”). UV radiation produced by the sun is commonly split into three bands: UV-A, with wavelengths of 320-400 nanometers; UV-B, with wavelengths of 320-280 nanometers; and UV-C, with wavelengths of 200-280 nanometers. Most people are familiar with UV light through the painful effects of sunburn. A small amount of UV radiation is necessary for the well-being of humans and other organisms because it promotes vitamin D synthesis in humans and acts as a germicide, controlling microbial growth. Too much UV radiation, however, is associated with many harmful effects.