The light is absorbed by melanin, the coloured pigment, present in the hair during the growth stage(anagen phase at which the bulb and the hair shaft is attached to the root), and is then conducted down to the route of the hair(hair shaft, hair follicle, epithelium and matrix). This light is turned into heat, and it is the heat that destroys the follicle and the route without side effects. This is ”selective photothermolysis”.