A measure of the character of light, where certain wavelength ranges define and distinguish different "types" of light. The photoreceptor cell detects and chemically converts light of certain wavelengths within the visible range (between 400-700 nanometers or nm) into electrical signals that travel, via the optic nerve, to the visual cortex in the brain.

Light is made of many bundles of energy (particles or quanta) and travels in a "wave" pattern. A wavelength (measured in nanometers) is the distance between the repeating "up-down-up" patterns (a sine wave) of the light wave-this is defined as a unit. Frequency, measured in Hertz, is the number of times that unit pattern repeats in a given time period. A frequency of 1 Hz means the wave repeats once per second. Light with shorter wavelengths tends to travel at a faster frequency and has greater energy, while longer wavelength light tends to cycle slower and be less energetic. Only light that travels at wavelengths within the visual spectrum, between 400-700 nm (at approximately 1015 Hz), is seen by the unaided eye.