Coherence, a fixed relationship between the phase of waves in a beam of radiation of a single frequency. Two beams of light are coherent when the phase difference between their waves is constant; they are noncoherent if there is a random or changing phase relationship.

What are the conditions for coherence?

In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference.

Where do you think coherence exists?

coherence exists in a sequence of words, sentences and paragraphs in which the reader can perceive connections... To produce a coherent stretch of discourse, writers use basic thought patterns, or logical patterns, in both simple and complex ways." (Brostoff, 1981).