Electromagnetic Spectrum

The electromagnetic spectrum is the range of

frequencies

(the

spectrum

) of

electromagnetic radiation

and their respective

wavelength

s and photon energies.

The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 10²⁵ hertz, corresponding to

wavelength

s from thousands of kilometers down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the

electromagnetic wave

s within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves,

microwave

s,

infrared

, visible light,

ultraviolet

,

X-ray

s, and gamma rays 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

itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length. Extreme ultraviolet, soft X-rays, hard X-rays and gamma rays are classified as ''ionizing radiation'' as their photons have enough energy to ionize atoms, causing chemical reactions.

In most of the frequency bands above, a technique called

spectroscopy

can be used to physically separate waves of different frequencies, producing a

spectrum

showing the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter. Other technological uses are described under

electromagnetic radiation

.

History and discovery

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 Greeks recognized that light traveled in straight lines and studied some of its properties, including 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

. 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

measured the

wavelength

of a light beam with his two-slit experiment thus conclusively demonstrating that light was a wave.

In 1800

William Herschel

discovered

infrared

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, 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

radiation.

The study of

electromagnetism

began in 1820 when

Hans Christian Ørsted

discovered that electric currents produce magnetic fields (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 Maxwell

developed four partial differential equations (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 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 waves. 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 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 telegraph and the

radio

.

In 1895

Wilhelm Röntgen

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

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 rays. In 1900

Paul Villard

was studying the radioactive emissions of

radium

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

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 only absorbed in discrete 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.

Range

Electromagnetic waves are typically described by any of the following three physical properties: the

frequency

''f'',

wavelength

λ

, or photon energy ''E''. Frequencies observed in astronomy range from (1 GeV gamma rays) down to the local plasma frequency 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

s, whereas wavelengths on the opposite end of the spectrum can be as long as the

universe

. Photon energy is directly proportional to the wave frequency, so gamma ray photons have the highest energy (around a billion electron volts), while radio wave photons have very low energy (around a femtoelectronvolt). 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 in a vacuum
* ''h'' = = is

Planck's constant

.

Whenever electromagnetic waves exist in a medium with matter, 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,

microwave

,

infrared

, visible light,

ultraviolet

,

X-ray

s and gamma rays. 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

(photon) it carries.

Spectroscopy

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. For example, many

hydrogen

atom

s emit a radio wave photon that has a wavelength of 21.12 cm. Also, frequencies of 30

Hz

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

The types of electromagnetic radiation are broadly classified into the following classes (regions, bands or types):
# Gamma radiation
# X-ray radiation
# Ultraviolet radiation
# Visible light
# Infrared radiation
# Microwave radiation
# Radio waves
This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation.

There are no precisely defined boundaries between the bands of the electromagnetic spectrum; rather they fade into each other like the bands in a rainbow (which is the sub-spectrum of visible light). Radiation of each frequency and wavelength (or in each band) has a mix of properties of the two regions of the spectrum that bound it. For example, red light resembles infrared radiation in that it can excite and add energy to some chemical bonds and indeed must do so to power the chemical mechanisms responsible for

photosynthesis

and the working of the visual system.

The distinction between X-rays and gamma rays is partly based on sources: the photons generated from nuclear decay or other nuclear and subnuclear/particle process are always termed gamma rays, whereas X-rays are generated by

electron

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 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), 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 sources (such as

gamma ray burst

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

-dependent (due to the

Doppler shift

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. 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 transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift 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.

Types of radiation

Radio waves

Radio

waves are emitted and received by antennas, which consist of conductors such as metal rod resonators. In artificial generation of radio waves, an electronic device called a transmitter generates an AC electric current which is applied to an antenna. The oscillating electrons in the antenna generate oscillating

electric

and

magnetic field

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

radio receiver

. Earth's atmosphere is mainly transparent to radio waves, except for layers of charged particles in the ionosphere which can reflect certain frequencies.

Radio waves are extremely widely used to transmit information across distances in radio communication systems such as radio broadcasting,

television

, two way radios,

mobile phone

s, communication satellites, and

wireless networking

. In a radio communication system, a radio frequency current is

modulated

with an information-bearing

signal

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 in the receiver. Radio waves are also used for navigation in systems like Global Positioning System (GPS) and navigational beacons, and locating distant objects in radiolocation and

radar

. They are also used for

remote control

, and for industrial heating.

The use of the radio spectrum is strictly regulated by governments, coordinated by the

International Telecommunication Union

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

Microwaves

Microwave

s are radio waves of short

wavelength

, from about 10 centimeters to one millimeter, in the SHF and EHF frequency bands. Microwave energy is produced with

klystron

and

magnetron

tubes, and with solid state devices such as Gunn and IMPATT diodes. 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

and

light

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

s, and for industrial heating and medical diathermy. Microwaves are the main wavelengths used in

radar

, and are used for satellite communication, and

wireless networking

technologies such as

Wi-Fi

. The copper cables (

transmission line

s) which are used to carry lower frequency radio waves to antennas have excessive power losses at microwave frequencies, and metal pipes called

waveguide

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 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'', 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.

Infrared radiation

The

infrared

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 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, reserving far infrared for wavelengths below 200 μm.
* Mid-infrared, from 30 to 120 THz (10–2.5 μm). Hot objects (

black-body

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. 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 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

would appear just beyond the opposite violet end.

Electromagnetic radiation with a

wavelength

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.

Ultraviolet radiation

Next in frequency comes

ultraviolet

(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

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'', 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

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). 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.

See also

* 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

* V band
* W band

Notes and references

External links

Australian Radiofrequency Spectrum Allocations Chart
(from Australian Communications and Media Authority)

Canadian Table of Frequency Allocations
(from Industry Canada)

U.S. Frequency Allocation Chart
– Covering the range 3 kHz to 300 GHz (from United States Department of Commerce, Department of Commerce)

UK frequency allocation table
(from Ofcom, which inherited the Radiocommunications Agency's duties, pdf format)

Flash EM Spectrum Presentation / Tool
– Very complete and customizable.

Poster "Electromagnetic Radiation Spectrum"
(992 kB)

{{Color topics

Electromagnetic spectrum,
Waves