Coherence of light refers to the phase relationship between one or more beams of light. This physical property is an important foundation that leads to interference between itself or other waves. The two forms of coherence that a wave can experience are temporal or spatial coherence. Spatial coherence refers to the distance between peaks or troughs, in other words, the phase of the wave at different points in time is the same, i.e the measure of the wave’s uniformity. Temporal coherence refers to a wave’s phase at various points along the direction of propagation, this tells us how monochromatic the wave is .
One important characteristic of light sources is how coherent they are, for example, the light produced from a well-designed laser will emit both spatially and temporally coherent. On the contrary, light from a light bulb is completely random and incoherent, but with a simple optical set up the incoherent light can be made coherent. Sunlight is also considered an incoherent source of light but under certain conditions.