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Light is radiant energy, usually referring to electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10 m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths. These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances. Various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm, children and young adults ultraviolet down to about 310 to 313 nm.
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 vacuum.
In common with all types of EMR, 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.
In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. This article focuses on visible light. See the electromagnetic radiation article for the general term.
- 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.”
- 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. 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."
- 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."
- Laufer, Gabriel (13 July 1996). Introduction to Optics and Lasers in Engineering. Cambridge University Press. p. 11. ISBN 978-0-521-45233-5. Retrieved 20 October 2013.
- Bradt, Hale (2004). Astronomy Methods: A Physical Approach to Astronomical Observations. Cambridge University Press. p. 26. ISBN 978-0-521-53551-9. Retrieved 20 October 2013.
- Ohannesian, Lena; Streeter, Anthony (9 November 2001). Handbook of Pharmaceutical Analysis. CRC Press. p. 187. ISBN 978-0-8247-4194-5. Retrieved 20 October 2013.
- Ahluwalia, V. K.; Goyal, Madhuri (1 January 2000). A Textbook of Organic Chemistry. Narosa. p. 110. ISBN 978-81-7319-159-6. Retrieved 20 October 2013.
- Sliney, David H.; Wangemann, Robert T.; Franks, James K.; Wolbarsht, Myron L. (1976). "Visual sensitivity of the eye to infrared laser radiation". Journal of the Optical Society of America 66 (4): 339–341. doi:10.1364/JOSA.66.000339. (subscription required (. "The foveal sensitivity to several near-infrared laser wavelengths was measured. It was found that the eye could respond to radiation at wavelengths at least as far as 1064 nm. A continuous 1064 nm laser source appeared red, but a 1060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina." ))
- Lynch, David K.; Livingston, William Charles (2001). Color and Light in Nature (2nd ed.). Cambridge, UK: Cambridge University Press. p. 231. ISBN 978-0-521-77504-5. Retrieved 12 October 2013. "Limits of the eye's overall range of sensitivity extends from about 310 to 1050 nanometers"
- Dash, Madhab Chandra; Dash, Satya Prakash (2009). Fundamentals Of Ecology 3E. Tata McGraw-Hill Education. p. 213. ISBN 978-1-259-08109-5. Retrieved 18 October 2013. "Normally the human eye responds to light rays from 390 to 760 nm. This can be extended to a range of 310 to 1,050 nm under artificial conditions."
- Saidman, Jean (15 May 1933). "Sur la visibilité de l'ultraviolet jusqu'à la longueur d'onde 3130" [The visibility of the ultraviolet to the wave length of 3130]. Comptes rendus de l'Académie des sciences (in French) 196: 1537–9.
- Gregory Hallock Smith (2006). Camera lenses: from box camera to digital. SPIE Press. p. 4. ISBN 978-0-8194-6093-6.
- Narinder Kumar (2008). Comprehensive Physics XII. Laxmi Publications. p. 1416. ISBN 978-81-7008-592-8.
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