Geometric Optics -- from Eric Weisstein's World of Physics

The regime of optics which ignores the effects of diffraction and considers light rays to travel in only straight lines. This assumption is valid when the wavelength is sufficiently small so that the path of rays can be described using only reflection and refraction. Three rays have particularly simple paths in geometric optics.

parallel ray: incident ray travels parallel to the principal axis, outgoing ray passes through the focus

principle focus ray: incident ray passes through the focus, the outgoing ray returns parallel to the principle axis

secondary axis ray: in a mirror, the incident ray is reflected along itself. In a lens, the ray passes through the optical center of the lens

When rays are nearly on-axis of optical elements, the paraxial approximation for small angles can be made, which greatly simplify the algebra. In addition to the paraxial approximation, the concept of focal plane may also be invoked, which hold that off-axis parallel rays are all focused to a single point in the plane containing the lens or mirror's focus.

Paraxial Approximation, Physical Optics