!Coherence refers to the degree of similarity between two or more wavefronts. In the context of planar wavefronts, coherence describes the extent to which the waves maintain a constant phase relationship as they propagate through space and time.
Coherence can be measured using various techniques, such as interferometry or spectral analysis. These methods involve comparing the phases and amplitudes of the wavefronts at different points in space or time, and calculating a coherence function or correlation coefficient.
Spatial coherence refers to the similarity of two wavefronts at different points in space, while temporal coherence describes the similarity of two wavefronts at different points in time. In other words, spatial coherence is concerned with the spatial distribution of the wavefronts, while temporal coherence is concerned with the time evolution of the wavefronts.
Coherence is important because it affects the interference and diffraction patterns of light waves. In applications such as imaging, holography, and optical communications, the coherence of the wavefronts determines the quality of the resulting image or signal.
Coherence can be controlled by manipulating the sources of the wavefronts, such as using lasers or spatial filters. Additionally, techniques such as phase shifting or filtering can be used to improve the coherence of the wavefronts. However, achieving perfect coherence is difficult in practice due to various factors such as environmental disturbances and imperfections in the optical system.