Why are two light waves sometimes incoherent?

Two light waves are sometimes incoherent because they do not maintain a constant phase relationship with each other.

In more detail, coherence refers to the correlation between the phases of two or more waves. When two light waves are coherent, they have a constant phase difference and the same frequency, which allows them to interfere constructively or destructively, producing an interference pattern. This is the principle behind many optical phenomena and technologies, such as lasers and holography.

However, two light waves are sometimes incoherent, meaning they do not maintain a constant phase relationship. This can occur for several reasons. Firstly, the light waves may originate from different sources, which are not phase-locked. For example, light from the sun or a light bulb is incoherent because it is produced by a multitude of independent atoms, each emitting light with its own random phase.

Secondly, even if the light waves originate from the same source, they can become incoherent if they travel different paths. This is because the phase of a light wave changes as it propagates, depending on the medium it travels through and its path length. If the path lengths or the properties of the media are not constant, the phase relationship between the light waves will change over time, making them incoherent.

Finally, the coherence of light can also be affected by the source's temporal stability. If the frequency of the light changes over time, the phase relationship between different parts of the wave will not be constant, resulting in temporal incoherence.

Incoherence is not necessarily a bad thing. In fact, it is essential for some applications. For instance, incoherent light is used in imaging systems, such as microscopes and telescopes, because it provides uniform illumination and avoids the formation of interference patterns, which could distort the image.