When sunlight, or artificially created ultraviolet light, contacts human skin, a tanning response takes place. Actually, this response is the body’s creation of a protective “umbrella” through darkening of the cells in the upper layer of the skin.
How this happens involves a little bit of physics and a little bit of photobiology, but don’t let that scare you- the concepts are easy enough to grasp.
It all starts when ultraviolet UV light contracts the skin. different components of the UV spectrum perform different functions. These components are:
UV-B: reddens the skin and initiates the tanning precess by stimulating the formation of melanin.