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The ray tracer will advance the ray over this distance, and then use a local derivative of the medium to calculate the ray's new direction. Ray tracing works by assuming that the particle or wave can be modeled as a large number of very narrow beams ( rays), and that there exists some distance, possibly very small, over which such a ray is locally straight. The ray is advanced by a small amount, and then the direction is re-calculated. Ray tracing of a beam of light passing through a medium with changing refractive index.
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Ray theory does not describe phenomena such as interference and diffraction, which require wave theory (involving the phase of the wave). When applied to problems of electromagnetic radiation, ray tracing often relies on approximate solutions to Maxwell's equations that are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength. More detailed analysis can be performed by using a computer to propagate many rays. Simple problems can be analyzed by propagating a few rays using simple mathematics. Ray tracing solves the problem by repeatedly advancing idealized narrow beams called rays through the medium by discrete amounts. Under these circumstances, wavefronts may bend, change direction, or reflect off surfaces, complicating analysis. In physics, ray tracing is a method for calculating the path of waves or particles through a system with regions of varying propagation velocity, absorption characteristics, and reflecting surfaces. Not to be confused with Ray casting or Ray tracing (graphics).