Light Explained

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight.[1] Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).[2] [3] This wavelength means a frequency range of roughly 430–750 terahertz (THz).




Electromagnetic spectrum and visible light

See main article: Electromagnetic spectrum.

Generally, EM radiation (the designation "radiation" excludes static electric, magnetic, and near fields), or EMR, is classified by wavelength into radio waves, microwaves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, and gamma rays.




Above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the very short (below 360 nm) ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses (such as insects and shrimp) are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light.

Various sources define visible light as narrowly as 420–680 nm[7] [8] to as broadly as 380–800 nm.[9] [10] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm;[11] children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm.[12] [13] [14]

Plant growth is also affected by the color spectrum of light, a process known as photomorphogenesis.

Speed of light

See main article: Speed of light.

The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approx. 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.




The effective velocity of light in various transparent substances containing ordinary matter, is less than in vacuum. For example, the speed of light in water is about 3/4 of that in vacuum.

Two independent teams of physicists were said to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Massachusetts, and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge.[17] However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped" it had ceased to be light.


See main article: Optics.



See main article: Refraction.


n1\sin\theta1=n2\sin\theta2 .

where θ1 is the angle between the ray and the surface normal in the first medium, θ2 is the angle between the ray and the surface normal in the second medium, and n1 and n2 are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.



Light sources

There are many sources of light. A body at a given temperature emits a characteristic spectrum of black-body radiation. A simple thermal source is sunlight, the radiation emitted by the chromosphere of the Sun at around 6000K peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units[18] and roughly 44% of sunlight energy that reaches the ground is visible.[19] Another example is incandescent light bulbs, which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in flames, but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum.




Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors.

Certain other mechanisms can produce light:


Units and measures

See main article: Photometry (optics) and Radiometry.



Light pressure

See main article: Radiation pressure. Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: photons strike and transfer their momentum. Light pressure is equal to the power of the light beam divided by c, the speed of light. Due to the magnitude of c, the effect of light pressure is negligible for everyday objects. For example, a one-milliwatt laser pointer exerts a force of about 3.3 piconewtons on the object being illuminated; thus, one could lift a U.S. penny with laser pointers, but doing so would require about 30 billion 1-mW laser pointers.[20] However, in nanometre-scale applications such as nanoelectromechanical systems (|NEMS), the effect of light pressure is more significant, and exploiting light pressure to drive NEMS mechanisms and to flip nanometre-scale physical switches in integrated circuits is an active area of research.[21] At larger scales, light pressure can cause asteroids to spin faster,[22] acting on their irregular shapes as on the vanes of a windmill. The possibility of making solar sails that would accelerate spaceships in space is also under investigation.[23] [24]

Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.[25] This should not be confused with the Nichols radiometer, in which the (slight) motion caused by torque (though not enough for full rotation against friction) is directly caused by light pressure.[26] As a consequence of light pressure, Einstein[27] in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter. He wrote, "radiation will exert pressure on both sides of the plate. The forces of pressure exerted on the two sides are equal if the plate is at rest. However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface. The backwardacting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief."

Historical theories about light, in chronological order

Classical Greece and Hellenism


In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. If the beam from the eye travels infinitely fast this is not a problem.[29]

In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote that "The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." (from On the nature of the Universe). Despite being similar to later particle theories, Lucretius's views were not generally accepted. Ptolemy (c. 2nd century) wrote about the refraction of light in his book Optics.[30]

Classical India






Particle theory




Wave theory

To explain the origin of colors, Robert Hooke (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 work Micrographia ("Observation IX"). In 1672 Hooke suggested that light's vibrations could be perpendicular to the direction of propagation. Christiaan Huygens (1629–1695) worked out a mathematical wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.[33]

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young). Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye. Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory. In 1816 André-Marie Ampère gave Augustin-Jean Fresnel an idea that the polarization of light can be explained by the wave theory if light were a transverse wave.[34]




Electromagnetic theory

See main article: Electromagnetic radiation.




Quantum theory



In February 2018, scientists reported, for the first time, the discovery of a new form of light, which may involve polaritons, that could be useful in the development of quantum computers.[38] [39]

See also

Notes and References

  1. [International Commission on Illumination|CIE]
  2. Book: Pal . G.K. . Pal . Pravati . Textbook of Practical Physiology . . 11 October 2013 . 1st . 2001 . Orient Blackswan . Chennai . 978-81-250-2021-9 . 387 . chapter 52 . 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. Book: Buser . Pierre A. . Imbert . Michel . Vision . 11 October 2013 . 1992 . MIT Press . 978-0-262-02336-8 . 50 . Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å..
  4. Book: Uzan, J-P . Leclercq . B . 2008 . The Natural Laws of the Universe: Understanding Fundamental Constants . The Natural Laws of the Universe: Understanding Fundamental Constants . 43–4 . 978-0-387-73454-5. . 10.1007/978-0-387-74081-2 .
  5. Book: Camera lenses: from box camera to digital . Gregory Hallock Smith . SPIE Press . 2006 . 978-0-8194-6093-6 . 4 .
  6. Book: Comprehensive Physics XII . Narinder Kumar . Laxmi Publications . 2008 . 978-81-7008-592-8 . 1416 .
  7. Book: Laufer, Gabriel . Introduction to Optics and Lasers in Engineering . Introduction to Optics and Lasers in Engineering . 20 October 2013 . 13 July 1996 . 978-0-521-45233-5 . 11. .
  8. Book: Bradt, Hale . Astronomy Methods: A Physical Approach to Astronomical Observations . 20 October 2013 . 2004 . Cambridge University Press . 978-0-521-53551-9 . 26.
  9. Book: Ohannesian . Lena . Streeter . Anthony . Handbook of Pharmaceutical Analysis . 20 October 2013 . 9 November 2001 . CRC Press . 978-0-8247-4194-5 . 187.
  10. Book: Ahluwalia . V.K. . Goyal . Madhuri . A Textbook of Organic Chemistry . 20 October 2013 . 1 January 2000 . Narosa . 978-81-7319-159-6 . 110.
  11. Sliney . David H. . Wangemann . Robert T. . Franks . James K. . Wolbarsht . Myron L. . 1976 . Visual sensitivity of the eye to infrared laser radiation . . 66 . 4 . 339–341 . 10.1364/JOSA.66.000339 . 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. . 1976JOSA...66..339S .
  12. Book: Lynch . David K. . Livingston . William Charles . Color and Light in Nature . 12 October 2013 . 2nd . 2001 . Cambridge University Press . Cambridge. 978-0-521-77504-5 . 231 . Limits of the eye's overall range of sensitivity extends from about 310 to 1050 nanometers.
  13. Book: Dash . Madhab Chandra . Dash . Satya Prakash . Fundamentals Of Ecology 3E . 18 October 2013 . 2009 . Tata McGraw-Hill Education . 978-1-259-08109-5 . 213 . 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..
  14. 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 . . 196 . 1537–9 . French .
  15. Scientific Method, Statistical Method and the Speed of Light . Statistical Science . 2000 . 15 . 254–278 . 3 . 10.1214/ss/1009212817 . 1847825. Oldford . R. W . MacKay . R. J .
  16. Michelson . A.A. . Measurements of the velocity of light between Mount Wilson and Mount San Antonio . Astrophysical Journal . January 1927 . 65 . 1 . 10.1086/143021 . 1927ApJ....65....1M.
  17. Web site: Harvard News Office . Harvard Gazette: Researchers now able to stop, restart light . . 2001-01-24 . 2011-11-08 . yes . . 28 October 2011 .
  18. Web site: Spectrum and the Color Sensitivity of the Eye . . 2017-08-29.
  19. Web site: Reference Solar Spectral Irradiance: Air Mass 1.5 . 2009-11-12.
  20. Tang . Hong . May The Force of Light Be With You . IEEE Spectrum . 1 October 2009 . 46 . 10 . 46–51 . 10.1109/MSPEC.2009.5268000.
  21. See, for example, nano-opto-mechanical systems research at Yale University.
  22. Web site: Asteroids Get Spun By the Sun . Kathy A. . Discover Magazine . 2004-02-05.
  23. Web site: Solar Sails Could Send Spacecraft 'Sailing' Through Space . NASA . 2004-08-31.
  24. Web site: NASA team successfully deploys two solar sail systems . NASA . 2004-08-09.
  26. Nichols . E.F . Hull . G.F. . 1903 . The Pressure due to Radiation . The Astrophysical Journal . 17 . 315–351 . 5 . 1903ApJ....17..315N . 10.1086/141035.
  27. Book: Einstein, A. . On the development of our views concerning the nature and constitution of radiation. Translated in: The Collected Papers of Albert Einstein, vol. 2 (Princeton University Press, Princeton, 1989) . 1909 . Princeton University Press . Princeton, New Jersey . 391.
  28. Book: Singh, S. . Fundamentals of Optical Engineering . 2009 . Discovery Publishing House . 9788183564366.
  29. Web site: Light through the ages: Ancient Greece to Maxwell . O'Connor . J J . Robertson . E F . August 2002.
  30. Book: Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary . Ptolemy and A. Mark Smith . Diane Publishing . 1996 . 23 . 978-0-87169-862-9.
  31. Web site: Shastra Pratibha 2015 Seniors Booklet . . 2017-08-29.
  32. Theories of light, from Descartes to Newton A.I. Sabra CUP Archive,1981 p. 48
  34. James R. Hofmann, André-Marie Ampère: Enlightenment and Electrodynamics, Cambridge University Press, 1996, p. 222.
  35. Book: Understanding Physics . David Cassidy . Gerald Holton . James Rutherford . Birkhäuser . 2002 . 978-0-387-98756-9 .
  36. Book: Longair, Malcolm . Theoretical Concepts in Physics . 2003 . 87.
  37. Book: Barrow, Gordon M. . Introduction to Molecular Spectroscopy . McGraw-Hill . 1962 . Scanned PDF . 62-12478.
  38. Web site: Hignett . Katherine . Physics Creates New Form Of Light That Could Drive The Quantum Computing Revolution . 16 February 2018 . Newsweek . 17 February 2018 .
  39. Liang, Qi-Yu. etal. Observation of three-photon bound states in a quantum nonlinear medium . 16 February 2018 . . 359 . 6377 . 783–786 . 10.1126/science.aao7293 . 29449489 . 1709.01478 . 2018Sci...359..783L .