In geometrical optics, light is treated as a bundle of rays. Monochromatic rays traveling through homogeneous media create straight-line paths that change direction only at surfaces that separate two different media. The laws of reflection and refraction define the way in which the path of each ray changes at such a surface. Today, the enormous numerical-processing power of computers is used in ray-tracing programs, in which the basic formulae of geometrical optics are used to calculate the propagation of a bundle of rays through an optical system. The design of microscope objectives, photo cameras, and telescopes is based largely on such calculations.
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