A coherent source is a source that emits a light wave with the same frequency, wavelength, and phase or has a constant phase difference). Coherent sources produce sustained interference patterns when the waves superimpose and the positions of maxima and minima are fixed. The interrelation between physical quantities of a single wave or between several waves is described by the property known as coherence. Coherent sources are produced by using prisms, lenses, and mirrors with specific specifications. Some of the techniques that help in the production of coherent sources are Fresnel’s biprism, Young’s double-slit experiment, and Lloyd’s mirror arrangement. Two waves with constant relative phase will be coherent). The amount of coherence can readily be measured by the interference visibility, which looks at the size of the interference fringes relative to the input waves). Coherence controls the visibility or contrast of interference patterns).