Abbreviated Hz. The unit of measurement which expresses the frequency of a periodic oscillation in cycles per second.

The optical reproduction of an object, produced by a lens or mirror. A typical positive lens converges rays to form a “real” image which can be photographed. A negative lens spreads rays to form a “virtual” image which can’t be projected.

A ray of light that falls on the surface of a lens — or any other object. The “angle of incidence” is the angle made by the ray with a perpendicular to the surface.