The branch of Physics which deals with the study of light and various phenomena associated with it is called Optics. In Geometrical Optics (or Ray Optics) we assume that light travels in a straight line in the form of rays. This model is useful for studying the formation of images by lenses and mirrors. On the other hand, in Physical Optics (or Wave Optics) light is assumed to be a wave. Various phenomena such as interference, diffraction and polarization, which cannot be explained by Ray Optics, are studied under Wave Optics.
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Pradyot 