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In Euclidean geometry, a ray, or half-line, given two distinct points A (the origin) and B on the ray, is set of points C on the line containing points A and B such that A is not strictly between C and B.

  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.

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