Editing Electromagnetic spectrum (section)

== History and discovery <span class="anchor" id="History"></span> ==
{{see also|History of electromagnetic theory|History of radio|History of electrical engineering|History of optics}}
Humans have always been aware of [[visible light]] and [[radiant heat]] but for most of history it was not known that these phenomena were connected or were representatives of a more extensive principle. The [[Ancient Greece|ancient Greeks]] recognized that light traveled in straight lines and studied some of its properties, including [[Reflection (physics)|reflection]] and [[refraction]]. Light was intensively studied from the beginning of the 17th century leading to the invention of important instruments like the [[telescope]] and [[microscope]]. [[Isaac Newton]] was the first to use the term ''[[spectrum]]'' for the range of colours that white light could be split into with a [[Prism (optics)|prism]]. Starting in 1666, Newton showed that these colours were intrinsic to light and could be recombined into white light. A debate arose over whether light had a wave nature or a particle nature with [[René Descartes]], [[Robert Hooke]] and [[Christiaan Huygens]] favouring a wave description and Newton favouring a particle description. Huygens in particular had a well developed theory from which he was able to derive the laws of reflection and refraction. Around 1801, [[Thomas Young (scientist)|Thomas Young]] measured the [[wavelength]] of a light beam with his [[Young's interference experiment|two-slit experiment]] thus conclusively demonstrating that light was a wave.

In 1800, [[William Herschel]] discovered [[infrared]] radiation.<ref>{{cite web|title=Herschel Discovers Infrared Light|url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html|archive-url=https://web.archive.org/web/20120225094516/http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html|archive-date=2012-02-25|work=Cool Cosmos Classroom activities|access-date=4 March 2013|quote=He directed sunlight through a glass prism to create a spectrum [...] and then measured the temperature of each colour. [...] He found that the temperatures of the colours increased from the violet to the red part of the spectrum. [...] Herschel decided to measure the temperature just ''beyond'' the red of the spectrum in a region where no sunlight was visible. To his surprise, he found that this region had the highest temperature of all.}}</ref> He was studying the temperature of different colours by moving a thermometer through light split by a prism. He noticed that the highest temperature was beyond red. He theorized that this temperature change was due to "calorific rays", a type of light ray that could not be seen. The next year, [[Johann Ritter]], working at the other end of the spectrum, noticed what he called "chemical rays" (invisible light rays that induced certain chemical reactions). These behaved similarly to visible violet light rays, but were beyond them in the spectrum.<ref>{{cite web|last=Davidson|first=Michael W.|title=Johann Wilhelm Ritter (1776–1810)|url=http://micro.magnet.fsu.edu/optics/timeline/people/ritter.html|publisher=The Florida State University|access-date=5 March 2013|quote=Ritter [...] hypothesized that there must also be invisible radiation beyond the violet end of the spectrum and commenced experiments to confirm his speculation. He began working with silver chloride, a substance decomposed by light, measuring the speed at which different colours of light broke it down. [...] Ritter [...] demonstrated that the fastest rate of decomposition occurred with radiation that could not be seen, but that existed in a region beyond the violet. Ritter initially referred to the new type of radiation as chemical rays, but the title of ultraviolet radiation eventually became the preferred term.}}</ref> They were later renamed [[ultraviolet]] radiation.

The study of [[electromagnetism]] began in 1820 when [[Hans Christian Ørsted]] discovered that [[electric current]]s produce [[magnetic field]]s ([[Oersted's law]]). Light was first linked to electromagnetism in 1845, when [[Michael Faraday]] noticed that the [[polarization of light]] traveling through a transparent material responded to a magnetic field (see [[Faraday effect]]). During the 1860s, [[James Clerk Maxwell]] developed four partial [[differential equation]]s ([[Maxwell's equations]]) for the [[electromagnetic field]]. Two of these equations predicted the possibility and behavior of waves in the field. Analyzing the speed of these theoretical waves, Maxwell realized that they must travel at a speed that was about the known [[speed of light]]. This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave. Maxwell's equations predicted an infinite range of frequencies of [[electromagnetic waves]], all traveling at the speed of light. This was the first indication of the existence of the entire electromagnetic spectrum.

Maxwell's predicted waves included waves at very low frequencies compared to infrared, which in theory might be created by oscillating charges in an ordinary [[Electrical Circuit|electrical circuit]] of a certain type. Attempting to prove Maxwell's equations and detect such low frequency electromagnetic radiation, in 1886, the physicist [[Heinrich Hertz]] built an apparatus to generate and detect what are now called [[radio wave]]s. Hertz found the waves and was able to infer (by measuring their wavelength and multiplying it by their frequency) that they traveled at the speed of light. Hertz also demonstrated that the new radiation could be both reflected and refracted by various [[Dielectric|dielectric media]], in the same manner as light. For example, Hertz was able to focus the waves using a lens made of tree [[pitch (resin)|resin]]. In a later experiment, Hertz similarly produced and measured the properties of [[microwave]]s. These new types of waves paved the way for inventions such as the [[Wireless telegraphy|wireless telegraph]] and the [[radio]].

In 1895, [[Wilhelm Röntgen]] noticed a new type of radiation emitted during an experiment with an [[Vacuum tube|evacuated tube]] subjected to a [[high voltage]]. He called this radiation "[[x-ray]]s" and found that they were able to travel through parts of the human body but were reflected or stopped by denser matter such as bones. Before long, many uses were found for this [[radiography]].

The last portion of the electromagnetic spectrum was filled in with the discovery of [[gamma ray]]s. In 1900, [[Paul Villard]] was studying the radioactive emissions of [[radium]] when he identified a new type of radiation that he at first thought consisted of particles similar to known [[Alpha particle|alpha]] and [[beta particle]]s, but with the power of being far more penetrating than either. However, in 1910, British physicist [[William Henry Bragg]] demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914, [[Ernest Rutherford]] (who had named them gamma rays in 1903 when he realized that they were fundamentally different from charged alpha and beta particles) and [[Edward Andrade]] measured their wavelengths, and found that gamma rays were similar to X-rays, but with shorter wavelengths.

The wave-particle debate was rekindled in 1901 when [[Max Planck]] discovered that light is absorbed only in discrete "[[Quantum|quanta]]", now called [[photon]]s, implying that light has a particle nature. This idea was made explicit by [[Albert Einstein]] in 1905, but never accepted by Planck and many other contemporaries. The modern position of science is that electromagnetic radiation has both a wave and a particle nature, the [[wave-particle duality]]. The contradictions arising from this position are still being debated by scientists and philosophers.