Engineering Physics

14.3 Coherence

To produce an interference, coherent sources are required. If two or more waves have the same wave length (or frequency) and they are in phase or maintain constant phase relationship then they are said to be coherent. Coherent sources can not be obtained from separate light sources but they are obtained from a single light source.

The coherence is of two types, namely (i) Temporal coherence and (ii) Spatial coherence.

(i) Temporal coherence: If the phase of an electromagnetic field at a point does not change at a given time then the field is said to be temporally coherent during that time. Suppose E(x, y ,z, t₁) is the field at time ‘t₁’ and E(x, y, z, t₂) is the field at a later time ‘t₂’. During this period if the phase is not ...