is the light falling on a subject before it has been reflected by that subject. Some light meters measure incident light and others measure reflected light. A camera's built-in metering system measures reflected light only.
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 (normal) to the surface. Index of Refraction For a specific material, the ratio of the speed of light in a vacuum (c) to the speed of light in the material. Because the speed of light in vacuum is faster than in any material, the index of refraction is always greater than 1.