Visible light is the portion of the electromagnetic spectrum between wavelengths of about 400 nm and 800 nm (in air). The different wavelength mixtures are interpreted by the human brain as colors, ranging from red at the lower frequency to violet at the highest. The three basic dimensions of light (and of all electromagnetic radiation) are brilliance (or amplitude), color (or frequency), and polarization (or angle of vibration).
Light propagates at a finite speed; even moving observers always measure the same value of c, the speed of light in vacuum. c = 299,792,458 metres per second. However when light passes through a transparent substance such as air, water or glass, its speed is reduced. 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 term light can also refer generally to electromagnetic radiation, e.g. ultraviolet light or infrared light. 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 intensity of light is often measured in Brightness temperature.
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