Light energy from the laser is converted into heat when the laser hits its target. Diode laser targets melanin, which pigments the hair (and why having a tan could burn your skin). When the melanin absorbs the laser light, it destroys the hair follicle. If the follicle is in the early, active (anagen) growth phase, it is more likely to be miniaturized permanently. If the skin is significantly lighter than the hair, the skin is far less affected and laser doesn’t “see” it, so the skin is left unaffected, and only the follicle is destroyed. The thicker the hair, the easier it is for the laser to “see”.