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Middle English, from Anglo-French rai, from Latin radius rod, ray


  • 1 a : any of the lines of light that appear to radiate from a bright object
b : a beam of radiant energy (as light) of small cross section
c (1) : a stream of material particles traveling in the same line (as in radioactive phenomena) (2) : a single particle of such a stream
  • 2 a : light cast by rays : radiance
b : a moral or intellectual light
  • 3 : a thin line suggesting a ray: as a : any of a group of lines diverging from a common center
b : half line
  • 4 a : one of the bony rods that extend and support the membrane in the fin of a fish
b : one of the radiating divisions of the body of a radiate animal (as a starfish)
  • 5 a : a branch or flower stalk of an umbel
b (1) : medullary ray (2) : vascular ray
c : ray flower


In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very complex optical systems to be analyzed mathematically or simulated by computer. Ray tracing uses 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. Ray theory does not describe phenomena such as interference and diffraction, which require wave theory (involving the phase of the wave).[1]

See also