Electromagnetic spectrum

Source: Wikipedia, the free encyclopedia.
blackbody temperature

The electromagnetic spectrum is the full range of

visible light, ultraviolet, X-rays, and gamma rays
. 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.

Radio waves, at the low-frequency end of the spectrum, have the lowest

kilometers, or more. They can be emitted and received by antennas
, and pass through the atmosphere, foliage, and most building materials.

Gamma rays, at the high-frequency end of the spectrum, have the highest photon energies and the shortest wavelengths—much smaller than an atomic nucleus. Gamma rays, X-rays, and extreme ultraviolet rays are called ionizing radiation because their high photon energy is able to ionize atoms, causing chemical reactions. Longer-wavelength radiation such as visible light is nonionizing; the photons do not have sufficient energy to ionize atoms.

Throughout most of the electromagnetic spectrum, spectroscopy can be used to separate waves of different frequencies, so that the intensity of the radiation can be measured as a function of frequency or wavelength. Spectroscopy is used to study the interactions of electromagnetic waves with matter.[1]

History and discovery

Humans have always been aware of

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,

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.[3] They were later renamed ultraviolet
radiation.

The study 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 microwaves. 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 this radiation "x-rays" 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

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 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

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

femtoelectronvolt
). These relations are illustrated by the following equations:

where:

Whenever electromagnetic waves travel 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

visible light, ultraviolet, X-rays 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.

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[5] and frequencies as high as 2.9×1027 Hz have been detected from astrophysical sources.[6]

Regions

The electromagnetic spectrum
A visualization of the electromagnetic spectrum.

The types of electromagnetic radiation are broadly classified into the following classes (regions, bands or types):[1]

  1. Gamma radiation
  2. X-ray radiation
  3. Ultraviolet radiation
  4. Visible light (light that humans can see)
  5. Infrared radiation
  6. Microwave radiation
  7. Radio waves

This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation.[1]

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

muonic atom transitions are also said to produce X-rays, even though their energy may exceed 6 megaelectronvolts (0.96 pJ),[10] whereas there are many (77 known to be less than 10 keV (1.6 fJ)) low-energy nuclear transitions (e.g., the 7.6 eV (1.22 aJ) 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.[11]

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

, 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

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.

Class   Wave-
length

Freq-
uency

Energy per
photon

Ionizing
radiation
γ Gamma rays   10 pm 30
EHz
124
keV
100 pm 3 EHz 12.4 keV
HX Hard X-rays
SX Soft X-rays 10 nm 30 PHz 124 eV
EUV Extreme
ultraviolet
121 nm 3 PHz 10.2 eV
  NUV
Near ultraviolet

400 nm 750 THz
  Visible spectrum 700 nm 480 THz
Infrared NIR Near infrared 1 μm 300
THz
1.24 eV
10 μm 30 THz 124 meV
MIR Mid infrared
100 μm 3 THz 12.4 meV
FIR Far infrared
1 mm 300
GHz
1.24 meV
Micro-
waves
EHF Extremely high
frequency
1 cm 30 GHz 124 μeV
SHF Super high
frequency
1 dm 3 GHz 12.4 μeV
UHF Ultra high
frequency
1 m 300
MHz
1.24 μeV
Radio
waves
VHF Very high
frequency
10 m 30 MHz 124 neV
HF High
frequency
100 m 3 MHz 12.4 neV
MF Medium
frequency
1 km 300
kHz
1.24 neV
LF Low
frequency
10 km 30 kHz 124
p
eV
VLF Very low
frequency
100 km 3 kHz 12.4 peV
ULF Ultra low
frequency
1
Mm
300 Hz 1.24 peV
SLF Super low
frequency
10 Mm 30 Hz 124
f
eV
ELF Extremely low
frequency
100 Mm 3 Hz 12.4 feV
Sources: File:Light spectrum.svg[12][13][14] Table shows the lower limits for the specified class

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 arising from these qualitative interaction differences.

Electromagnetic radiation interaction with matter
Region of the spectrum Main interactions with matter
Radio Collective oscillation of charge carriers in bulk material (plasma oscillation). An example would be the oscillatory travels of the electrons in an antenna.
Microwave through far infrared Plasma oscillation, molecular rotation
Near infrared Molecular vibration, plasma oscillation (in metals only)
Visible Molecular electron excitation (including pigment molecules found in the human retina), plasma oscillations (in metals only)
Ultraviolet Excitation of molecular and atomic valence electrons, including ejection of the electrons (photoelectric effect)
X-rays Excitation and ejection of core atomic electrons, Compton scattering (for low atomic numbers)
Gamma rays Energetic ejection of core electrons in heavy elements, Compton scattering (for all atomic numbers), excitation of atomic nuclei, including dissociation of nuclei
High-energy gamma rays Creation of
particle-antiparticle pairs
. At very high energies a single photon can create a shower of high-energy particles and antiparticles upon interaction with matter.

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 alternating electric current which is applied to an antenna. The oscillating electrons in the antenna generate oscillating electric and magnetic fields 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

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

Plot of Earth's atmospheric opacity to various wavelengths of electromagnetic radiation. This is the surface-to-space opacity, the atmosphere is transparent to longwave radio transmissions within the troposphere but opaque to space due to the ionosphere.
Plot of atmospheric opacity for terrestrial to terrestrial transmission showing the molecules responsible for some of the resonances

wireless networking technologies such as Wi-Fi. The copper cables (transmission lines) which are used to carry lower-frequency radio waves to antennas have excessive power losses at microwave frequencies, and metal pipes called waveguides
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 gases limits practical propagation distances to a few kilometers.

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.[15]
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:[1]

Visible light

sRGB rendering of the spectrum of visible light
sRGB rendering of the spectrum of visible light
Colour
nm
)
Frequency
(THz)
Photon energy
(eV)
  violet
380–450 670–790 2.75–3.26
  blue
450–485 620–670 2.56–2.75
  cyan
485–500 600–620 2.48–2.56
  green
500–565 530–600 2.19–2.48
  yellow
565–590 510–530 2.10–2.19
  orange
590–625 480–510 1.98–2.10
  red
625–750 400–480 1.65–1.98

Above infrared in frequency comes

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 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 colours 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

The amount of penetration of UV relative to altitude in Earth's ozone

Next in frequency comes ultraviolet (UV). In frequency (and thus energy), UV rays sit between the violet end of the visible spectrum and the X-ray range. The UV wavelength spectrum ranges from 399 nm to 10 nm and is divided into 3 sections: UVA, UVB, and UVC.

UV is the lowest energy range energetic enough to ionize atoms, separating electrons from them, and thus causing chemical reactions. UV, X-rays, and gamma rays are thus collectively called ionizing radiation; exposure to them can damage living tissue. UV can also cause substances to glow with visible light; this is called fluorescence. UV fluorescence is used by forensics to detect any evidence like blood and urine, that is produced by a crime scene. Also UV fluorescence is used to detect counterfeit money and IDs, as they are laced with material that can glow under UV.

At the middle range of UV, UV rays cannot ionize but can break chemical bonds, making molecules unusually reactive.

thymine dimers making it a very potent mutagen
. Due to skin cancer caused by UV, the sunscreen industry was invented to combat UV damage. Mid UV wavelengths are called UVB and UVB lights such as germicidal lamps are used to kill germs and also to sterilize water.

The Sun emits 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

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

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 areal density of 1000 g/cm2), equivalent to 10 meters thickness of water.[17]
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

radiation cancer therapy.[18] More commonly, gamma rays are used for diagnostic imaging in nuclear medicine, an example being PET scans. The wavelength of gamma rays can be measured with high accuracy through the effects of Compton scattering
.

See also

Notes and references

  1. ^ a b c d e f Mehta, Akul (25 August 2011). "Introduction to the Electromagnetic Spectrum and Spectroscopy". Pharmaxchange.info. Retrieved 2011-11-08.
  2. ^ "Herschel Discovers Infrared Light". Cool Cosmos Classroom activities. Archived from the original on 2012-02-25. Retrieved 4 March 2013. 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.
  3. ^ Davidson, Michael W. "Johann Wilhelm Ritter (1776–1810)". The Florida State University. Retrieved 5 March 2013. 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.
  4. (PDF) on 2017-10-01. Direct link to value.
  5. ^ Condon, J. J.; Ransom, S. M. "Essential Radio Astronomy: Pulsar Properties". National Radio Astronomy Observatory. Archived from the original on 2011-05-04. Retrieved 2008-01-05.
  6. S2CID 17886934
    .
  7. .
  8. .
  9. .
  10. ^ Corrections to muonic X-rays and a possible proton halo slac-pub-0335 (1967)
  11. ^ "Gamma-Rays". Hyperphysics.phy-astr.gsu.edu. Retrieved 2010-10-16.
  12. UC Davis
    lecture slides
  13. ^ Elert, Glenn. "The Electromagnetic Spectrum". The Physics Hypertextbook. Retrieved 2022-01-21.
  14. ^ Stimac, Tomislav. "Definition of frequency bands (VLF, ELF... etc.)". vlf.it. Retrieved 2022-01-21.
  15. ^ "Advanced weapon systems using lethal Short-pulse terahertz radiation from high-intensity-laser-produced plasmas". India Daily. March 6, 2005. Archived from the original on 6 January 2010. Retrieved 2010-09-27.
  16. ^ "Reference Solar Spectral Irradiance: Air Mass 1.5". Retrieved 2009-11-12.
  17. ^ Koontz, Steve (26 June 2012) Designing Spacecraft and Mission Operations Plans to Meet Flight Crew Radiation Dose. NASA/MIT Workshop. See pages I-7 (atmosphere) and I-23 (for water).
  18. ^ Uses of Electromagnetic Waves | gcse-revision, physics, waves, uses-electromagnetic-waves | Revision World

External links