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.
O----O-----*--->
A B C
Geometric optics
In geometric optics a ray or a (light) beam 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