This article has provided a general description of what a ray is. In summary:

  • Rays are lines that model energy propagation through media such as glass.
  • Rays have position and direction, and may also have associated phase, amplitude, and polarization data.
  • The direction of a ray is normal to the wavefront (surfaces of constant phase) in isotropic media. Energy flows along the ray direction.
  • The amplitude of the ray indicates how much energy or power the wavefront has over some local area.
  • The optical path length of a ray indicates the phase of the electric field, which is useful for performing wavefront aberration and diffraction calculations.
  • Collections of rays, such as grids of rays, may be used to model entire wavefronts. Each ray in the collection is associated with a fininte amount of power, energy, or flux.
References

1. Shannon, R. R., The Art and Science of Optical Design, Cambridge University Press.
2. Bass, Michael, Handbook of Optics Volume I, McGraw Hill