Catch a Wave:

Joseph M. Geary, Center for Applied Optics, University of Alabama-Huntsville.

Light travels in straight lines, as evidenced by shadow-casting phenomenon, and the pinhole camera. The pinhole camera also helps define a "ray" of light. A point source emits light rays in all directions. A group of these rays would have a certain optical path length: length ? refractive index. The optical path length is a radius in this case, and the ray tips lie on the surface of a sphere whose center is the point source.
This surface is a basic example of what is called a wavefront. (Rays and their associated wavefronts are always orthogonal to one another.) Examining that wavefront can tell us much about any optics or an optical system that is between the light source and the measured wavefront. For example, NASA used several types of wavefront sensors to test the Hubble Space Telescope and its associated instruments.