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A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) get separated.

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight.<ref>CIE (1987). International Lighting Vocabulary. Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7.
By the International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.”</ref> Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−11 to 7.00 × 10−11 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).<ref name="Pal2001">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name="BuserImbert1992">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> This wavelength means a frequency range of roughly 430–750 terahertz (THz).

The main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence. For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.

The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299 792 458 meters per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in a vacuum.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of light, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.


Light sections
Intro  Electromagnetic spectrum and visible light   Speed of light   Optics  Units and measures  Light pressure  Historical theories about light, in chronological order  See also  Notes  References  External links  

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