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 

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.

See also

• diffraction
• Glossary of Riemannian and metric geometry#R for its meaning in Riemannian geometry.