TheInfoList

The electromagnetic spectrum is the range of
frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The SI base unit, base unit of time in the Internation ...

(the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

) of
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

and their respective
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

s and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one
hertz The hertz (symbol: Hz) is the unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action ...
to above 1025 hertz, corresponding to
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

s from thousands of
kilometer The kilometre (SI symbol: km; or ), spelt kilometer in American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the Eng ...
s down to a fraction of the size of an
atomic nucleus The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...
. This frequency range is divided into separate bands, and the
electromagnetic wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

s within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are:
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
s,
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

s,
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

,
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
,
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

,
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

s, and
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s at the high-frequency (short wavelength) end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. The limit for long wavelengths is the size of the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

itself, while it is thought that the short wavelength limit is in the vicinity of the
Planck length In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
. Extreme ultraviolet, soft X-rays, hard X-rays and gamma rays are classified as ''
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of s or s that have sufficient to s or s by detaching s from them. The particles generally travel at a speed that is greater than 1% of , and the electromagnetic w ...
'' as their photons have enough energy to
ionize Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...
atoms, causing chemical reactions. In most of the frequency bands above, a technique called
spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

can be used to physically separate waves of different frequencies, producing a
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

showing the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter. Other technological uses are described under
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

.

# History and discovery

Humans have always been aware of
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
and
radiant heat Thermal radiation is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space an ...
but for most of history it was not known that these phenomena were connected or were representatives of a more extensive principle. The
ancient Greeks Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, antiquity ( AD 600). This era was ...
recognized that light traveled in straight lines and studied some of its properties, including
reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal r ...
and
refraction In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...

. Light was intensively studied from the beginning of the 17th century leading to the invention of important instruments like the
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

and
microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a laboratory instrument used to examine objects that are too small to be seen by the naked eye Naked eye, also called bare ...

.
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

was the first to use the term ''spectrum'' for the range of colours that white light could be split into with a
prism A prism A prism An optical prism is a transparent optics, optical element with flat, polished surfaces that refraction, refract light. At least one surface must be angled—elements with two parallel surfaces are not prisms. The traditional ge ...

. 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 René Descartes ( or ; ; Latinized Latinisation or Latinization can refer to: * Latinisation of names, the practice of rendering a non-Latin name in a Latin style * Latinisation in the Soviet Union, the campaign in the USSR during the 1920s ...

,
Robert Hooke Robert Hooke FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resources ...
and
Christiaan Huygens Christiaan Huygens ( , also , ; la, Hugenius; 14 April 1629 – 8 July 1695), also spelled Huyghens, was a Dutch mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) i ...

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, measured the
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

of a light beam with his two-slit experiment thus conclusively demonstrating that light was a wave. In 1800
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

discovered
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

radiation. He was studying the temperature of different colors 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 Johann Wilhelm Ritter (16 December 1776 – 23 January 1810) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of German ...
, 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. They were later renamed
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

electromagnetism Electromagnetism is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in ...

began in 1820 when
Hans Christian Ørsted Hans Christian Ørsted ( , ; often rendered Oersted in English; 14 August 17779 March 1851) was a Danish physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge ...

discovered that electric currents produce magnetic fields (
Oersted's law In electromagnetism Electromagnetism is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the o ...
). Light was first linked to electromagnetism in 1845, when
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

noticed that the polarization of light traveling through a transparent material responded to a
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

(see
Faraday effect The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), ...

). During the 1860s
James MaxwellJames Maxwell may refer to: Arts and entertainment *James Maxwell (actor) (1929–1995), American actor and theatre director *Jim Maxwell (commentator) (born 1950), Australian sports commentator *Jimmy Maxwell (bandleader) (born 1953), musician an ...

developed four partial differential equations (
Maxwell's equations Maxwell's equations are a set of coupled partial differential equation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), ...
) for the
electromagnetic field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the in ...
. 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 The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
. This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave.
Maxwell's equations Maxwell's equations are a set of coupled partial differential equation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), ...
predicted an infinite range of frequencies of
electromagnetic waves In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through ...

, all traveling at the speed of light. This was the first indication of the existence of the entire electromagnetic
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

. 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 of a certain type. Attempting to prove Maxwell's equations and detect such low frequency electromagnetic radiation, in 1886 the physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empi ...

built an apparatus to generate and detect what are now called
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
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 media, in the same manner as light. For example, Hertz was able to focus the waves using a lens made of tree
resin In polymer chemistry Polymer chemistry is a sub-discipline of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, stru ...
. In a later experiment, Hertz similarly produced and measured the properties of
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

s. These new types of waves paved the way for inventions such as the
wireless telegraph Wireless telegraphy or radiotelegraphy is transmission of telegraph Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object beari ...
and the
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

. In 1895
Wilhelm Röntgen Wilhelm Conrad Röntgen (; ; 27 March 184510 February 1923) was a German mechanical engineer Mechanical may refer to: Machine * Mechanical system, a system that manages the power of forces and movements to accomplish a task * Machine (mechanica ...

noticed a new type of radiation emitted during an experiment with an evacuated tube subjected to a high voltage. He called these radiations
x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

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 Radiography is an imaging technique using X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 ...
. The last portion of the electromagnetic spectrum was filled in with the discovery of
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s. In 1900
Paul Villard Paul Ulrich Villard (28 September 1860 – 13 January 1934) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a ...

was studying the radioactive emissions of
radium Radium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

when he identified a new type of radiation that he first thought consisted of particles similar to known alpha and beta particles, but with the power of being far more penetrating than either. However, in 1910, British physicist
William Henry Bragg Sir William Henry Bragg (2 July 1862 – 12 March 1942) was an English physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method ...

demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914,
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist A physicist is a scientist A scientist is a person who conducts scientific research The sci ...
(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 Edward Neville da Costa Andrade FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States ...
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 Max Karl Ernst Ludwig Planck, (; ; 23 April 1858 – 4 October 1947) was a Germans, German theoretical physicist whose discovery of quantum mechanics, energy quanta won him the Nobel Prize in Physics in 1918. Planck made many substantial co ...

discovered that light is only absorbed in discrete quanta, now called
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

s, implying that light has a particle nature. This idea was made explicit by
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the theory of relativity The theo ...

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 . The contradictions arising from this position are still being debated by scientists and philosophers.

# Range

Electromagnetic waves are typically described by any of the following three physical properties: the
frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

''f'',
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

, or
photon energy Photon energy is the energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, i ...
''E''. Frequencies observed in astronomy range from (1
GeV GEV may refer to: * ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games * Ashe County Airport, in North Carolina, United States * Gällivare Lapland Airport, in Sweden * Generalized extreme value distribution In probability theory ...
gamma rays) down to the local
plasma frequency Plasma oscillations, also known as Langmuir waves (after Irving Langmuir Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Lati ...
of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to the wave frequency, so gamma rays have very short wavelengths that are fractions of the size of
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s, whereas wavelengths on the opposite end of the spectrum can be as long as the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

.
Photon energy Photon energy is the energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, i ...
is directly proportional to the wave frequency, so gamma ray photons have the highest energy (around a billion
electron volt In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through S ...
s), while radio wave photons have very low energy (around a
femto ''Femto'' (symbol f) is a unit prefix A unit prefix is a specifier or mnemonic that is prepended to units of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity ...
electronvolt). These relations are illustrated by the following equations: :$f = \frac, \quad\text\quad f = \frac, \quad\text\quad E=\frac,$ where: * ''c'' = is the
speed of light The speed of light in vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called paramet ...
in a vacuum * ''h'' = = is . Whenever electromagnetic waves exist in a
medium Medium may refer to: Science and technology Aviation *Medium bomber, a class of war plane *Tecma Medium, a French hang glider design Communication * Media (communication), tools used to store and deliver information or data * Medium of i ...
with
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
, their wavelength is decreased. Wavelengths of electromagnetic radiation, whatever medium they are traveling through, are usually quoted in terms of the ''vacuum wavelength'', although this is not always explicitly stated. Generally, electromagnetic radiation is classified by wavelength into
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
,
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

,
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

,
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
,
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

,
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

s and
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per
quantum In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

(photon) it carries.
Spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

can detect a much wider region of the EM spectrum than the visible wavelength range of 400 nm to 700 nm in a vacuum. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. Spectroscopes are widely used in
astrophysics Astrophysics is a science that employs the methods and principles of physics in the study of astronomical objects and phenomena. Among the subjects studied are the Sun, other stars, galaxy, galaxies, extrasolar planets, the interstellar medium and ...
. For example, many
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

s emit a
radio wave Radio waves are a type of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
photon that has a wavelength of 21.12 cm. Also, frequencies of 30 and below can be produced by and are important in the study of certain stellar nebulae and frequencies as high as have been detected from astrophysical sources.

# Regions

chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyda ...
s and indeed must do so to power the chemical mechanisms responsible for
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

and the working of the
visual system The visual system comprises the sensory organ A sense is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined b ...
. The distinction between X-rays and gamma rays is partly based on sources: the photons generated from
nuclear decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus The atomic nucleus is the small, dense region consisting of proton A ...
or other nuclear and subnuclear/particle process are always termed gamma rays, whereas X-rays are generated by
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

ic transitions involving highly energetic inner atomic electrons. In general, nuclear transitions are much more energetic than electronic transitions, so gamma rays are more energetic than X-rays, but exceptions exist. By analogy to electronic transitions,
muonic atom An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same Electric charge, charge. For example, electrons may be replaced by other negatively charged particles such as mu ...
transitions are also said to produce X-rays, even though their energy may exceed , whereas there are many (77 known to be less than ) low-energy nuclear transitions (''e.g.'', the nuclear transition of
thorium-229m Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the general ...
), and, despite being one million-fold less energetic than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear origin. The convention that EM radiation that is known to come from the nucleus is always called "gamma ray" radiation is the only convention that is universally respected, however. Many astronomical
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
sources (such as
gamma ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant Galaxy, galaxies. They are the brightest and most energetic Electromagnetic radiation, electromagnetic events known to occur in ...

s) are known to be too energetic (in both intensity and wavelength) to be of nuclear origin. Quite often, in high energy physics and in medical radiotherapy, very high energy EMR (in the > 10 MeV region)—which is of higher energy than any nuclear gamma ray—is not called X-ray or gamma ray, but instead by the generic term of "high energy photons". The region of the spectrum where a particular observed electromagnetic radiation falls is
reference frame In physics, a frame of reference (or reference frame) consists of an abstract coordinate system In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, wi ...

-dependent (due to the
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer (physics), observer who is moving relative to the wave source. It is named after the Austrian physicist ...

for light), so EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum. For example, consider the
cosmic microwave background The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang theory A theory is a rational Rationality is the quality or state of being rational – that is, being based on or agreeable to reason Reason is the capacity ...
. It was produced when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from
Lyman series In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Ph ...
transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological
red shift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...
to put it into the microwave region of the spectrum for observers moving slowly (compared to the speed of light) with respect to the cosmos.

## Rationale for names

Electromagnetic radiation interacts with matter in different ways across the spectrum. These types of interaction are so different that historically different names have been applied to different parts of the spectrum, as though these were different types of radiation. Thus, although these "different kinds" of electromagnetic radiation form a quantitatively continuous spectrum of frequencies and wavelengths, the spectrum remains divided for practical reasons related to these qualitative interaction differences.

Radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

waves are emitted and received by
antennas In radio engineering Broadcast engineering is the field of electrical engineering, and now to some extent computer engineering and information technology, which deals with radio and television broadcasting. Audio engineering and RF engineer ...
, which consist of conductors such as metal rod
resonator A resonator is a device or system that exhibits resonance Resonance describes the phenomenon of increased amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Pe ...
s. In artificial generation of radio waves, an electronic device called a
transmitter In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particle ...
generates an AC
electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, ...
which is applied to an antenna. The oscillating electrons in the antenna generate oscillating
electric Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...

and
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

s that radiate away from the antenna as radio waves. In reception of radio waves, the oscillating electric and magnetic fields of a radio wave couple to the electrons in an antenna, pushing them back and forth, creating oscillating currents which are applied to a

. Earth's atmosphere is mainly transparent to radio waves, except for layers of charged particles in the
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
which can reflect certain frequencies. Radio waves are extremely widely used to transmit information across distances in
radio communication Radio is the technology of signaling and communicating Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Organization, groups through the use ...
systems such as
,
television Television, sometimes shortened to TV or telly, is a telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Gre ...

,
two way radio A two-way radio is a radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They a ...
s,
mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunication Telecommunication is the tra ...

s,
communication satellite Communication (from Latin ''communicare'', meaning "to share") is the act of developing meaning among entities or groups through the use of sufficiently mutually understood signs, symbol A symbol is a mark, sign, or word that indicates, ...
s, and
wireless networking A wireless network is a computer network A computer network is a set of computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern ...

modulated In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplifi ...

with an information-bearing
signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image processing, images, and scientific measurements. Sig ...

in a transmitter by varying either the amplitude, frequency or phase, and applied to an antenna. The radio waves carry the information across space to a receiver, where they are received by an antenna and the information extracted by
demodulation Demodulation is extracting the original information-bearing signal from a carrier wave upright=1.4, The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is sign ...
Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government The federal government of the United States (U.S. federal government or U.S. governme ...
(GPS) and
navigational beacon A beacon is an intentionally conspicuous device designed to attract attention to a specific location. A common example is the lighthouse A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lam ...
s, and locating distant objects in
radiolocation Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the location In geography Geography (from Ancient Greek, Greek: , ''geographia'', literally "earth description") is a field of science devoted t ...
and
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

. They are also used for
remote control In electronics, a remote control (also known as a remote or clicker) is an electronic device used to operate another device from a distance, usually wirelessly. In consumer electronics, a remote control can be used to operate devices such as ...

, and for industrial heating. The use of the
radio spectrum The radio spectrum is the part of the electromagnetic spectrum with Frequency, frequencies from 3 hertz, Hz to 300 Hertz, GHz. Electromagnetic waves in this frequency range, called ''radio waves'', are widely used in modern technology, ...
is strictly regulated by governments, coordinated by the
International Telecommunication Union The International Telecommunication Union is a list of specialized agencies of the United Nations, specialized agency of the United Nations responsible for all matters related to information and communications technology, information and co ...

(ITU) which allocates frequencies to different users for different uses.

## Microwaves

Microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

s are radio waves of short
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

, from about 10 centimeters to one millimeter, in the SHF and EHF frequency bands. Microwave energy is produced with
klystron A klystron is a specialized linear-beam vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particles, such as e ...

and
magnetron Image:Magnetron2.jpg, Magnetron with section removed to exhibit the cavities. The cathode in the center is not visible. The antenna emitting microwaves is at the left. The magnets producing a field parallel to the long axis of the device are not ...

tubes, and with solid state devices such as
GunnGunn may refer to: Places * Gunn City, Missouri, a village * Gunn, Northern Territory, outer suburb of Darwin * Gunn, Alberta, Canada, a hamlet * Gunn Valley, a mountain valley in British Columbia, Canada * Gun Lake (British Columbia), a Canadia ...
and
IMPATT diode An IMPATT diode (impact ionization avalanche transit-time diode) is a form of high-power semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume res ...
s. Although they are emitted and absorbed by short antennas, they are also absorbed by polar molecules, coupling to vibrational and rotational modes, resulting in bulk heating. Unlike higher frequency waves such as
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

and
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

which are absorbed mainly at surfaces, microwaves can penetrate into materials and deposit their energy below the surface. This effect is used to heat food in
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven upA double oven A ceramic oven An oven is a tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment. ...

s, and for industrial heating and medical
diathermy Diathermy is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical therapy and in surgical procedures. The earliest observations on the reactions of high-frequency electromagnetic currents upon the ...
. Microwaves are the main wavelengths used in
radar Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (''ranging''), angle, or velocity of objects. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor ...

, and are used for
satellite communication A communications satellite is an artificial satellite alt=, A full-size model of the Earth observation satellite ERS 2 ">ERS_2.html" ;"title="Earth observation satellite ERS 2">Earth observation satellite ERS 2 In the context of spacefli ...
, and
wireless networking A wireless network is a computer network A computer network is a set of computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern ...

technologies such as
Wi-Fi Wi-Fi () is a family of wireless network A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking is a method by which homes, telecommunications networks and business installa ...

. The copper cables (
transmission line In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch o ...

s) which are used to carry lower frequency radio waves to antennas have excessive power losses at microwave frequencies, and metal pipes called
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities dec ...

s are used to carry them. Although at the low end of the band the atmosphere is mainly transparent, at the upper end of the band absorption of microwaves by atmospheric gasses limits practical propagation distances to a few kilometers.
Terahertz radiation Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic wave In physics Physics is the ...
or sub-millimeter radiation is a region of the spectrum from about 100 GHz to 30 terahertz (THz) between microwaves and far infrared which can be regarded as belonging to either band. Until recently, the range was rarely studied and few sources existed for microwave energy in the so-called ''
terahertz gap __NOTOC__ In engineering, the terahertz gap is a frequency band A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spe ...
'', but applications such as imaging and communications are now appearing. Scientists are also looking to apply terahertz technology in the armed forces, where high-frequency waves might be directed at enemy troops to incapacitate their electronic equipment. Terahertz radiation is strongly absorbed by atmospheric gases, making this frequency range useless for long-distance communication.

The
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

part of the electromagnetic spectrum covers the range from roughly 300 GHz to 400 THz (1 mm – 750 nm). It can be divided into three parts: * Far-infrared, from 300 GHz to 30 THz (1 mm – 10 μm). The lower part of this range may also be called microwaves or terahertz waves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by
phonons In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...
in solids. The water in Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere in effect opaque. However, there are certain wavelength ranges ("windows") within the opaque range that allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as
Submillimetre astronomy Submillimetre astronomy or submillimeter astronomy (see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from , , "through" and , , "I sp ...
, reserving far infrared for wavelengths below 200 μm. * Mid-infrared, from 30 to 120 THz (10–2.5 μm). Hot objects (
black-body A black body or blackbody is an idealized physical body In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a defined contiguous boundary in three-dimen ...

radiators) can radiate strongly in this range, and human skin at normal body temperature radiates strongly at the lower end of this region. This radiation is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the ''fingerprint region'', since the mid-infrared absorption spectrum of a compound is very specific for that compound. * Near-infrared, from 120 to 400 THz (2,500–750 nm). Physical processes that are relevant for this range are similar to those for visible light. The highest frequencies in this region can be detected directly by some types of photographic film, and by many types of solid state image sensors for infrared photography and videography.

## Visible light

Above infrared in frequency comes
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
. The Sun emits its peak power in the visible region, although integrating the entire emission power spectrum through all wavelengths shows that the Sun emits slightly more infrared than visible light. By definition, visible light is the part of the EM spectrum the luminosity function, human eye is the most sensitive to. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. This action allows the chemical mechanisms that underlie human vision and plant photosynthesis. The light that excites the human
visual system The visual system comprises the sensory organ A sense is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined b ...
is a very small portion of the electromagnetic spectrum. A rainbow shows the optical (visible) part of the electromagnetic spectrum; infrared (if it could be seen) would be located just beyond the red side of the rainbow whilst
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

would appear just beyond the opposite violet end. Electromagnetic radiation with a
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

between 380 nanometre, nm and 760 nm (400–790 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up into the several colors of light observed in the visible spectrum between 400 nm and 780 nm. If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes the eyes, this results in visual perception of the scene. The brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this insufficiently-understood psychophysical phenomenon, most people perceive a bowl of fruit. At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and technology can also manipulate a broad range of wavelengths. Optical fiber transmits light that, although not necessarily in the visible part of the spectrum (it is usually infrared), can carry information. The modulation is similar to that used with radio waves.

Next in frequency comes
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

(UV). The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray. UV is the longest wavelength radiation whose photons are energetic enough to ionization, ionize atoms, separating
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

s from them, and thus causing chemical reactions. Short wavelength UV and the shorter wavelength radiation above it (X-rays and gamma rays) are called ''
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of s or s that have sufficient to s or s by detaching s from them. The particles generally travel at a speed that is greater than 1% of , and the electromagnetic w ...
'', and exposure to them can damage living tissue, making them a health hazard. UV can also cause many substances to glow with visible light; this is called ''fluorescence''. At the middle range of UV, UV rays cannot ionize but can break chemical bonds, making molecules unusually reactive. Sunburn, for example, is caused by the disruptive effects of middle range UV radiation on Human skin, skin Cell (biology), cells, which is the main cause of skin cancer. UV rays in the middle range can irreparably damage the complex DNA molecules in the cells producing thymine dimers making it a very potent mutagen. The Sun emits significant UV radiation (about 10% of its total power), including extremely short wavelength UV that could potentially destroy most life on land (ocean water would provide some protection for life there). However, most of the Sun's damaging UV wavelengths are absorbed by the atmosphere before they reach the surface. The higher energy (shortest wavelength) ranges of UV (called "vacuum UV") are absorbed by nitrogen and, at longer wavelengths, by simple diatomic oxygen in the air. Most of the UV in the mid-range of energy is blocked by the ozone layer, which absorbs strongly in the important 200–315 nm range, the lower energy part of which is too long for ordinary dioxygen in air to absorb. This leaves less than 3% of sunlight at sea level in UV, with all of this remainder at the lower energies. The remainder is UV-A, along with some UV-B. The very lowest energy range of UV between 315 nm and visible light (called UV-A) is not blocked well by the atmosphere, but does not cause sunburn and does less biological damage. However, it is not harmless and does create oxygen radicals, mutations and skin damage.

## X-rays

After UV come
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

s, which, like the upper ranges of UV are also ionizing. However, due to their higher energies, X-rays can also interact with matter by means of the Compton scattering, Compton effect. Hard X-rays have shorter wavelengths than soft X-rays and as they can pass through many substances with little absorption, they can be used to 'see through' objects with 'thicknesses' less than that equivalent to a few meters of water. One notable use is diagnostic X-ray imaging in medicine (a process known as
radiography Radiography is an imaging technique using X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 ...
). X-rays are useful as probes in high-energy physics. In astronomy, the accretion disks around neutron stars and black holes emit X-rays, enabling studies of these phenomena. X-rays are also emitted by stellar corona and are strongly emitted by some types of nebulae. However, X-ray telescopes must be placed outside the Earth's atmosphere to see astronomical X-rays, since the great depth of the atmosphere of Earth is opaque to X-rays (with area density, areal density of 1000 g/cm2), equivalent to 10 meters thickness of water. This is an amount sufficient to block almost all astronomical X-rays (and also astronomical gamma rays—see below).

## Gamma rays

After hard X-rays come gamma rays, which were discovered by Paul Ulrich Villard in 1900. These are the most energetic photons, having no defined lower limit to their wavelength. In astronomy they are valuable for studying high-energy objects or regions, however as with X-rays this can only be done with telescopes outside the Earth's atmosphere. Gamma rays are used experimentally by physicists for their penetrating ability and are produced by a number of radioisotopes. They are used for irradiation of foods and seeds for sterilization, and in medicine they are occasionally used in Radiation oncology, radiation cancer therapy.Uses of Electromagnetic Waves , gcse-revision, physics, waves, uses-electromagnetic-waves , Revision World
/ref> More commonly, gamma rays are used for diagnostic imaging in nuclear medicine, an example being Positron emission tomography, PET scans. The wavelength of gamma rays can be measured with high accuracy through the effects of Compton scattering.

* Bandplan * Cosmic ray * Digital dividend after digital television transition * Electroencephalography * Infrared window * Ionizing radiation * List of international common standards * Optical window * Ozone layer * Radiant energy * Radiation * Radio window *
Spectroscopy Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...

* V band * W band