A source of light that produces light waves of the same wavelength with the same frequency and in the same phase. Two independent light sources cannot be coherent sources. An example of a coherent light source is a laser in contrast to incoherent sources of light, such as the sun, a lamp, or a candle. Two coherent sources can be two parallel slit-lamps lighted by a laser beam behind them. Coherent sources are needed to observe the effects of optical phenomena.