This article is also available in Japanese.

Rays are used to model the propagation of light through optical systems. For homogeneous, isotropic media, such as common optical glass, rays are straight lines. Rays are normal to the local wavefront and point along the direction of energy flow. Rays have position, direction, amplitude, phase, and possibly polarization data associated with them.

At surfaces between different types of media, rays can refract, reflect, or diffract. The interface between different media will generally alter the direction and other properties of the ray.

ZEMAX uses ray tracing to model the propagation of light from the source point, through the optical system, and on to the final image surface. The resulting distribution of amplitude, phase, and polarization of a collection of rays can be used to predict a wide range of optical phenomena.

The ray model is a very convenient, powerful, and accurate means for propagating light.

When tracing through inhomogeneous media, such as gradient index glass, rays will in general follow curved paths. Some materials, such as uniaxial crystals, are not isotropic, and within these types of media rays are not normal to the wavefront. Both of these cases are handled by ZEMAX and are described in other articles. Polarization ray tracing is a complex subject and is also described in a separate article.