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Ray optics (or geometrical optics) describes light propagation in terms of rectilinear paths in homogeneous media. The simple concepts of ray optics are very powerful in explaining image formation by mirrors, lenses, and the eye and in a multitude of optical instruments. The design of photographic objectives, which may consist of a dozen lenses to optimize image quality, is done within the framework of ray optics. However, we know that ray optics does not account for all phenomena in the propagation of light. Diffraction and interference cannot be understood without taking into account the wave nature of light.