incoherent light

Incoherent light refers to light where the phase relationship between waves is random or constantly changing. This is in contrast to coherent light, where the phase relationship is fixed. While strictly incoherent light is an approximation, real-world sources like sunlight are not completely incoherent.12

The concept of partial coherence was developed to describe light that falls between these two extremes. Experiments, such as Young's double-slit experiment, demonstrate that interference patterns, a hallmark of wave behavior, are observable only when light sources are coherent. The visibility of interference fringes, a measure of coherence, decreases as the separation between light sources increases, eventually disappearing when the light becomes effectively incoherent. Lasers are an example of sources that produce highly coherent light.231