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Definition: A line (usually a straight line) constructed perpendicular to successive wavefronts to indicate the path of travel of light. Rays undergo refraction and reflection at boundaries in a straightforward way, so they are useful in geometrical optics where it is not necessary to resort to a full wave model of the light.
Source: The Penguin Dictionary of Science. London: Penguin, 2009. Credo Reference. Web. 17 August 2012.