In many situations light can be understood using a simple model based on rays and wave fronts. A ray is a beam of light that travels in a straight line. A wave front is the line (not necessarily straight) or surface connecting all the light that left a source at the same time. For a source like the Sun, rays radiate out in all directions; the wave fronts are spheres centered on the Sun. If the source is a long way away, the wave fronts can be treated as parallel lines.

Rays and wave fronts can represent light when light interacts with objects much larger than the wavelength of light, which is about 500 nm. Because of this, the analysis of light interacting with objects like mirrors or lenses is based on the geometry of similar triangles. This is why this branch of optics is known as geometrical optics.