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O----O-----*---> A B C
In geometric optics a ray is a curve describing the direction in which light or other electromagnetic radiation is propagated. The ray in geometric optics is perpendicular to the wavefront in physical optics[?].
In most simple cases, light rays within a given medium are straight lines. Light passing from one medium to another undergoes refraction or total internal reflection following Snell's law.
See also:
wikipedia.org dumped 2003-03-17 with terodump