Electromagnetic spectrum
The electromagnetic spectrum is the full range of
. 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
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
In 1800,
The study of
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
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
The wave-particle debate was rekindled in 1901 when
Range
Electromagnetic waves are typically described by any of the following three physical properties: the
where:
- c = 299792458 m/s is the speed of light in vacuum
- h = 6.62607015×10−34 J·s = 4.13566733(10)×10−15 eV·s is the Planck constant.[4]
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
Regions
The types of electromagnetic radiation are broadly classified into the following classes (regions, bands or types):[1]
- Gamma radiation
- X-ray radiation
- Ultraviolet radiation
- Visible light (light that humans can see)
- Infrared radiation
- Microwave radiation
- 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
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
The region of the spectrum where a particular observed electromagnetic radiation falls is
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.
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
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
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]
- 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 THz to 120 THz (10–2.5 μm). Hot objects (black-bodyradiators) 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 THz 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
Colour
|
nm )
|
Frequency (THz) |
Photon energy (eV) |
---|---|---|---|
380–450 | 670–790 | 2.75–3.26 | |
450–485 | 620–670 | 2.56–2.75 | |
485–500 | 600–620 | 2.48–2.56 | |
500–565 | 530–600 | 2.19–2.48 | |
565–590 | 510–530 | 2.10–2.19 | |
590–625 | 480–510 | 1.98–2.10 | |
625–750 | 400–480 | 1.65–1.98 |
Above infrared in frequency comes
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
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.
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
X-rays
After UV come
Gamma rays
After hard X-rays come
See also
Notes and references
- ^ a b c d e f Mehta, Akul (25 August 2011). "Introduction to the Electromagnetic Spectrum and Spectroscopy". Pharmaxchange.info. Retrieved 2011-11-08.
- ^ "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.
- ^ 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.
- doi:10.1103/RevModPhys.80.633. Archived from the original(PDF) on 2017-10-01. Direct link to value.
- ^ 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.
- S2CID 17886934.
- ISBN 978-0-201-02116-5.
- ISBN 978-0-12-436603-9.
- ISBN 978-3-540-25312-9.
- ^ Corrections to muonic X-rays and a possible proton halo slac-pub-0335 (1967)
- ^ "Gamma-Rays". Hyperphysics.phy-astr.gsu.edu. Retrieved 2010-10-16.
- UC Davislecture slides
- ^ Elert, Glenn. "The Electromagnetic Spectrum". The Physics Hypertextbook. Retrieved 2022-01-21.
- ^ Stimac, Tomislav. "Definition of frequency bands (VLF, ELF... etc.)". vlf.it. Retrieved 2022-01-21.
- ^ "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.
- ^ "Reference Solar Spectral Irradiance: Air Mass 1.5". Retrieved 2009-11-12.
- ^ 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).
- ^ Uses of Electromagnetic Waves | gcse-revision, physics, waves, uses-electromagnetic-waves | Revision World
External links
- Australian Radiofrequency Spectrum Allocations Chart (from Australian Communications and Media Authority)
- Canadian Table of Frequency Allocations Archived 2008-12-09 at the Industry Canada)
- U.S. Frequency Allocation Chart – Covering the range 3 kHz to 300 GHz (from Department of Commerce)
- UK frequency allocation table (from Radiocommunications Agency's duties, pdf format)
- Flash EM Spectrum Presentation / Tool – Very complete and customizable.
- Poster "Electromagnetic Radiation Spectrum" (992 kB)