Temporal coherence is the measure of the average correlation between the value of a wave and itself delayed by τ τ , at any pair of times. Temporal coherence tells us how monochromatic a source is. In other words, it characterizes how well a wave can interfere with itself at a different time. The delay over which the phase or amplitude wanders by a significant amount (and hence the correlation decreases by significant amount) is defined as the coherence time τ τ c . At a delay of τ τ = 0 the degree of coherence is perfect, whereas it drops significantly as the delay passes τ τ = τ τ c . The coherence length L c is defined as the distance the wave travels in time τ τ c .