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Light ○｜Definition｜1st｜20251119205401-00-⌔
Light
Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye.12 Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahertz. The visible band sits adjacent to the infrared (with longer wavelengths and lower frequencies) and the ultraviolet (with shorter wavelengths and higher frequencies), called collectively optical radiation.34
In physics, the term “light” may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not.56 In this sense, gamma rays, X-rays, microwaves and radio waves are also light. This is why visible radiation is commonly termed visible light.78
The primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization. Its speed in vacuum, 299 792 458 m/s, is one of the fundamental constants of nature.9 All electromagnetic radiation exhibits some properties of both particles and waves. Single, massless elementary particles, or quanta, of light called photons can be detected with specialized equipment; phenomena like interference are described by waves. Most everyday interactions with light can be understood using geometrical optics. Quantum 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.
Printed 2026-06-28.
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Link to original Footnotes
CIE (1987). International Lighting Vocabulary Archived 27 February 2010 at the Wayback Machine. Number 17.4. CIE, 4th ed.. 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.” ↩CIE JTC 8. “light,
noun”. CIE S 017/E:2020 ILV: International Lighting Vocabulary, 2nd Edition. International Commission on Illumination (CIE). doi:10.25039/S017.2020. ↩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. ↩
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. ↩
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. ↩
Narinder Kumar (2008). Comprehensive Physics XII. Laxmi Publications. p. 1416. ISBN 978-81-7008-592-8. ↩
SI Brochure, Appendix 2: Mise en pratique for the definition of the metre in the SI (PDF) (9 ed.). International Bureau of Weights and Measures. 20 May 2019. p. 2. Retrieved 20 December 2025. ↩
“Spectroscopy: A Measurement Powerhouse”. nist.gov. National Institute of Standards and Technology. 16 April 2025. Retrieved 20 December 2025. ↩
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. 43–44. Bibcode:2008nlu..book…U. doi:10.1007/978-0-387-74081-2 (inactive 1 July 2025). ISBN 978-0-387-73454-5. ↩
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