For other meanings see: Light, South Australia[?].
Visible light is that portion of the spectrum between the wavelengths of about 400 nm and 800 nm (in air). Light can also be characterized by its frequency. The frequency and wavelength of light obey the relation
where <math>\lambda</math> is the wavelength, <math>f</math> is the frequency, <math>c</math> is the speed of light, and <math>n</math> is the refractive index of the material through which the light is passing.
All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as <math>c = 299,792,458</math> metres per second; however, when light passes through a transparent substance such as air, water or glass, its speed is reduced, and it suffers refraction. Thus, <math>n=1</math> in a vacuum and <math>n>1</math> in matter. It is a violation of the technical terminology of physics to speak of the "velocity of light;" velocity is reserved for a different use.
The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.
The different wavelengths are interpreted by the human brain as colors, ranging from red at the longest wavelengths (lowest frequencies) to violet at the shortest wavelengths (highest frequencies). The intervening frequencies are seen as orange, yellow, green, blue, and, conventionally, indigo. The frequencies of the spectrum immediately outside the range the human eye is able to perceive are called ultraviolet (UV) at the high frequency end and infrared (IR) at the low. Though humans cannot see IR, we do perceive it by receptors in the skin as heat. Cameras that can pick up IR and convert it to visible light are called night-vision cameras. UV radiation is not perceived by humans at all except in a very delayed fashion, as overexposure of the skin to UV light causes sunburn, or skin cancer. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see IR using pits in their heads.
The following quantities and units are used to measure light.