# Light

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

Light or visible light is electromagnetic radiation that can be perceived by the human eye.[1] Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahertz, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).[2][3]

In physics, the term "light" may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not.[4][5] In this sense, gamma rays, X-rays, microwaves and radio waves are also light. The primary properties of light are intensity, propagation direction, frequency or wavelength spectrum and polarization. Its speed in vacuum, 299792458 m/s, is one of the fundamental constants of nature.[6] Like all types of electromagnetic radiation, visible light propagates by massless elementary particles called photons that represents the quanta of electromagnetic field, and can be analyzed as both waves and particles. The study of light, known as optics, is an important research area in modern physics.

The main source of natural light on Earth is the Sun. 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.

1. ^ CIE (1987). International Lighting Vocabulary Archived 27 February 2010 at the Wayback Machine. 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."
2. ^ Pal, G.K.; Pal, Pravati (2001). "chapter 52". Textbook of Practical Physiology (1st ed.). Chennai: Orient Blackswan. p. 387. ISBN 978-81-250-2021-9. Archived from the original on 8 October 2022. Retrieved 11 October 2013. The human eye has the ability to respond to all the wavelengths of light from 400–700 nm. This is called the visible part of the spectrum.
3. ^ Buser, Pierre A.; Imbert, Michel (1992). Vision. MIT Press. p. 50. ISBN 978-0-262-02336-8. Retrieved 11 October 2013. Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.
4. ^ Gregory Hallock Smith (2006). Camera lenses: from box camera to digital. SPIE Press. p. 4. ISBN 978-0-8194-6093-6. Archived from the original on 8 October 2022. Retrieved 15 November 2020.
5. ^ Narinder Kumar (2008). Comprehensive Physics XII. Laxmi Publications. p. 1416. ISBN 978-81-7008-592-8.
6. ^ Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe: Understanding Fundamental Constants. Translated by Robert Mizon. Springer-Praxis, Internet Archive: 2020-06-14 AbdzexK uban. pp. 43–4. Bibcode:2008nlu..book.....U. doi:10.1007/978-0-387-74081-2. ISBN 978-0-387-73454-5.