What is the characteristic of light referred to as 'coherent'?

The characteristic of light referred to as 'coherent' is that it remains in phase over a distance. Coherence implies a consistent phase relationship between the waves of light, meaning that the peaks and troughs of the waves align consistently as they travel. This characteristic is essential in applications such as lasers, where coherent light can be focused more precisely and used in various technologies, including communication and imaging.

In contrast, other characteristics mentioned, such as variation in phase, imply that the light waves do not maintain a stable phase relationship, resulting in less defined interference patterns. Characteristics like being colored or dispersed relate to the wavelength and spread of light, but do not define coherence in the same way that a consistent phase does. Coherence, therefore, specifically requires that the light waves maintain a constant phase relationship, which enhances their utility in various optical applications.