Where does ray collide?

In geometry, a ray is the part of a line that begins at a single endpoint and extends without limit in one direction, so it has a start but no finish. The idea descends from the lines and extensions of Euclid's Elements and underlies modern definitions of angles, which are formed by two rays sharing an endpoint. The referent is an abstract one-directional half-line with no width or substance, and it is unrelated to a beam of light or a flat-bodied fish beyond the shared meaning of something issuing from a point.[1]

Unit

A half-line defined by an endpoint and a direction

Recovery means

Not applicable

Measured by

Specified by its endpoint and direction; pairs of rays define and measure angles

In geometrical optics, a ray is an idealised line drawn perpendicular to the wavefronts of light, representing the direction in which energy flows and tracing the path light follows as it is reflected and refracted. Born and Wolf's Principles of optics derives ray behaviour as the short-wavelength limit of wave optics, the regime in which lenses and mirrors can be designed by tracing rays through them. The referent is a geometric stand-in for the flow of light energy, valid when wavelengths are small compared with the optics, and it borrows the half-line of pure geometry to model a physical phenomenon.[2]

Unit

The direction and path of light energy through an optical system

Recovery means

Not applicable

Measured by

Followed by ray tracing through surfaces using Snell's law, yielding image position, aberrations, and spot size