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The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. The existence of the hypothetical substance luminiferous aether proposed by Huygens in 1678 was cast into strong doubt in the late nineteenth century by the Michelson–Morley experiment. Antognozzi, M.; Bermingham, C. R.; Harniman, R. L.; Simpson, S.; Senior, J.; Hayward, R.; Hoerber, H.; Dennis, M. R.; Bekshaev, A. Y. (August 2016). "Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever". Nature Physics. 12 (8): 731–735. arXiv: 1506.04248. Bibcode: 2016NatPh..12..731A. doi: 10.1038/nphys3732. ISSN 1745-2473. S2CID 52226942. The behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths. When EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Generally, electromagnetic radiation (EMR) is classified by wavelength into radio waves, microwaves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays and gamma rays. The designation " radiation" excludes static electric, magnetic and near fields.

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 1,050 nm; [11] children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm. [12] [13] [14] Main article: Refraction Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle. 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. Bibcode: 1976JOSA...66..339S. doi: 10.1364/JOSA.66.000339. PMID 1262982. 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 1,064 nm. A continuous 1,064 nm laser source appeared red, but a 1,060 nm pulsed laser source appeared green, which suggests the presence of second harmonic generation in the retina. Michelson, A.A. (January 1927). "Measurements of the velocity of light between Mount Wilson and Mount San Antonio". Astrophysical Journal. 65: 1. Bibcode: 1927ApJ....65....1M. doi: 10.1086/143021. Barrow, Gordon M. (1962). Introduction to Molecular Spectroscopy (Scanned PDF). McGraw-Hill. LCCN 62-12478.Visible light" redirects here. For all parts of the electromagnetic spectrum that can be seen by the eye, see Visible spectrum. Our eyes react to light. When we see something, we see the light it reflects, or the light it gives off. For example, a lamp gives off light. Everything else in the room the lamp is in reflects the lamp's light. As the viewer, one cannot directly determine where the ray of light came from prior to reflecting off an object. Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. Berns, Roy S. (2019). Billmeyer and Saltzman's Principles of Color Technology. Fred W. Billmeyer, Max Saltzman (4th ed.). Hoboken, NJ: Wiley. ISBN 978-1-119-36668-3. OCLC 1080250734. To explain the origin of colours, 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 aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. [36] Christiaan Huygens Thomas Young's sketch of a double-slit experiment showing diffraction. Young's experiments supported the theory that light consists of waves.

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. [18] 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. Svitil, Kathy A. (5 February 2004). "Asteroids Get Spun By the Sun". Discover Magazine. Archived from the original on 9 October 2012 . Retrieved 8 May 2007. Later, Fresnel independently worked out his own wave theory of light and presented it to the Académie des Sciences in 1817. Siméon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favor of the wave theory, helping to overturn Newton's corpuscular theory. [ dubious – discuss] By the year 1821, Fresnel was able to show via mathematical methods that polarization could be explained by the wave theory of light if and only if light was entirely transverse, with no longitudinal vibration whatsoever. [ citation needed] 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. Some species of animals generate their own light, a process called bioluminescence. For example, fireflies use light to locate mates and vampire squid use it to hide themselves from prey.Light moves in a straight line. The straight line path is often drawn as a ray. Humans can only see rays of light traveling directly into the eyes. Ray diagrams are used to illustrate light traveling from one place to another. A beam of light can be thought of as a set of light rays. EMR in the visible light region consists of quanta (called photons) that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans (infrared) because its photons no longer have enough individual energy to cause a lasting molecular change (a change in conformation) in the visual molecule retinal in the human retina, which change triggers the sensation of vision.

Albert: This is called a reflection! So now we know that light can come from different sources, and that some objects are transparent, some are opaque and cast a shadow, and some objects reflect light!

Light sources

a b c "Shastra Pratibha 2015 Seniors Booklet" (PDF). Sifuae.com. Archived from the original (PDF) on 30 May 2015 . Retrieved 29 August 2017. Harvard News Office (24 January 2001). "Harvard Gazette: Researchers now able to stop, restart light". News.harvard.edu. Archived from the original on 28 October 2011 . Retrieved 8 November 2011. By the International Lighting Vocabulary, the definition of light is: "Any radiation capable of causing a visual sensation directly." The fact that light could be polarized was for the first time qualitatively explained by Newton using the particle theory. Étienne-Louis Malus in 1810 created a mathematical particle theory of polarization. Jean-Baptiste Biot in 1812 showed that this theory explained all known phenomena of light polarization. At that time the polarization was considered as the proof of the particle theory. David Cassidy; Gerald Holton; James Rutherford (2002). Understanding Physics. Birkhäuser. ISBN 978-0-387-98756-9. Archived from the original on 8 October 2022 . Retrieved 15 November 2020.

Léon Foucault carried out an experiment which used rotating mirrors to obtain a value of 298 000 000 m/s [16] in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mount Wilson to Mount San Antonio in California. The precise measurements yielded a speed of 299 796 000 m/s. [17] Laufer, Gabriel (1996). "Geometrical Optics". Introduction to Optics and Lasers in Engineering. p. 11. Bibcode: 1996iole.book.....L. doi: 10.1017/CBO9781139174190.004. ISBN 978-0-521-45233-5 . Retrieved 20 October 2013.Reference Solar Spectral Irradiance: Air Mass 1.5". Archived from the original on 12 May 2019 . Retrieved 12 November 2009. In physics colour is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. The radiation of such wavelengths constitutes that portion of the electromagnetic spectrum known as the visible spectrum—i.e., light. Ptolemy and A. Mark Smith (1996). Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary. Diane Publishing. p. 23. ISBN 978-0-87169-862-9. White light is made up of all the different colors of light added together. When white light shines through a prism, it splits up into different colors, becoming a spectrum. The spectrum contains all of the wavelengths of light that we can see. Red light has the longest wavelength, and violet (purple) light has the shortest.