Light can be described as a wave, but when its wavelength is much smaller than the objects and openings involved, its propagation can often be approximated geometrically using wavefronts and rays.

Wavefronts

A wavefront is a surface of equal phase. For a point source in a uniform medium, wavefronts are approximately spherical. Far from the source, a small region can be approximated by parallel plane wavefronts.

Rays

A ray is drawn perpendicular to the wavefront and points in the local direction of propagation.

Rays are not physical particles. They are a geometric representation that becomes useful when diffraction can be neglected.

Changing medium

Within a uniform medium, straight rays represent uniform propagation. At an interface, a wave can reflect, transmit and change direction because its speed changes.

Limits of ray optics

Ray optics explains mirrors, lenses and many imaging systems efficiently. When openings or structures become comparable to the wavelength, interference and diffraction become important and the full wave description is required.

Wavefronts provide the bridge between the wave picture of light and the ray picture of geometrical optics.