Light is a form of energy produced by many chemical reactions. The sun and other stars produce huge amounts of heat and light through fusion and are the source of most visible light in the universe. Other reactions that produce light are combustion and chemoluminescence.
The basic unit of light is called a photon and can act as both a particle and a wave. Photons are produced when a reaction causes an electron to move from a higher nuclear orbital to a lower one, releasing energy in the process. Light energy and heat energy are often produced in the same reactions. Because of a principle called black-body radiation, some substances emit visible light at high temperatures, which is why very hot metal glows red or white.
Sunlight takes a little more than eight minutes to travel from the Sun to Earth, and the power of sunlight is roughly 120 watts per square meter on Earth. The Sun produces ultraviolet and infrared light in addition to the visible spectrum. Ultraviolet light has a shorter wavelength than visible light and causes sunburns, while infrared light has a longer wavelength and is used in night vision devices.
The candela measures wattage over an area and is the official unit of light intensity. An average candle has one candela of luminous intensity.