Electromagnetic energy is a term used to describe all the different kinds of energies released into space by stars such as the Sun. These kinds of energies include some that you will recognize and some that will sound strange. They include:
An electromagnetic wave has a few important properties:
* In fact, each of these waves actually has a range of frequencies. For instance, FM radio waves actually range from 87.5 to 108.0 MHz.
[ Radio Waves ]
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communications satellites, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight. Radio waves travel at the speed of light in a vacuum. If radio waves strike an electrically conductive object of any size, they are slowed according to that object's permeability and permittivity.
The wavelength is the distance from one 'peak' of magnetic flux to the next, or the peak of one 'wave' to the next, and is inversely proportional to the frequency. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters which is the wavelength of a 1 hertz radio signal. A 1 megahertz radio signal has a wavelength of 299.8 meters. Radio frequency (RF) energy has been used in medical treatments for over 75 years generally for minimally invasive surgeries and coagulation, including the treatment of sleep apnea. Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.
The wavelength is the distance from one 'peak' of magnetic flux to the next, or the peak of one 'wave' to the next, and is inversely proportional to the frequency. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters which is the wavelength of a 1 hertz radio signal. A 1 megahertz radio signal has a wavelength of 299.8 meters. Radio frequency (RF) energy has been used in medical treatments for over 75 years generally for minimally invasive surgeries and coagulation, including the treatment of sleep apnea. Magnetic resonance imaging (MRI) uses radio frequency waves to generate images of the human body.
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| Diagram of the electric fields (E) and magnetic fields (H) of radio waves emitted by a monopole radio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular as implied by the phase diagram in the lower right. |
[ TV Waves ]
[ Radar Waves ]
[ Heat (infrared radiation) ]
Infrared (IR) light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 0.74 micrometres (µm) to 300 µm. This range of wavelengths corresponds to a frequency range of approximately 1 to 400 THz, and includes most of the thermal radiation emitted by objects near room temperature. Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. The existence of infrared radiation was first discovered in 1800 by astronomer William Herschel.
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| An image of two people in mid-infrared ("thermal") light (false-color |
Much of the energy from the Sun arrives on Earth in the form of infrared radiation. Sunlight at zenith provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth's climate. Infrared light is used in industrial, scientific, and medical
applications. Night-vision devices using infrared illumination allow
people or animals to be observed without the observer being detected. In
astronomy, imaging at infrared wavelengths allows observation of
objects obscured by interstellar dust. Infrared imaging cameras are used
to detect heat loss in insulated systems, to observe changing blood
flow in the skin, and to detect overheating of electrical apparatus. Infrared imaging is used extensively for military and civilian purposes. Military applications include target acquisition, surveillance, night vision, homing and tracking. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, remote temperature sensing, short-ranged wireless communication, spectroscopy, and weather forecasting. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect objects such as planets, and to view highly red-shifted objects from the early days of the universe.
Humans at normal body temperature radiate chiefly at wavelengths around 10 μm (micrometers), as shown by Wien's displacement law. At the atomic level, infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared energy range, based on their frequency and intensity.
[ Light ]
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| The Sun is Earth's primary source of light. About 44% of the sun's electromagnetic radiation that reaches the ground is in the visible light range. |
Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has a wavelength in the range of about 380 nanometres to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths. Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second (about 300,000 kilometers per second), is one of the fundamental constants
of nature. Visible light, as with all types of electromagnetic
radiation (EMR), is experimentally found to always move at this speed in
vacuum. In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.
In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. The speed of light in a vacuum is defined to be exactly 299,792,458 m/s
(approximately 186,282 miles per second). The fixed value of the speed
of light in SI units results from the fact that the metre is now defined
in terms of the speed of light. All forms of electromagnetic radiation
are believed to move at exactly this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo
attempted to measure the speed of light in the seventeenth century. An
early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io.
Noting discrepancies in the apparent period of Io's orbit, he
calculated that light takes about 22 minutes to traverse the diameter of
Earth's orbit. However, its size was not known at that time. If Rømer had known the
diameter of the Earth's orbit, he would have calculated a speed of
227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel
was placed in the path of the light beam as it traveled from the
source, to the mirror and then returned to its origin. Fizeau found that
at a certain rate of rotation, the beam would pass through one gap in
the wheel on the way out and the next gap on the way back. Knowing the
distance to the mirror, the number of teeth on the wheel, and the rate
of rotation, Fizeau was able to calculate the speed of light as
313,000,000 m/s. Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson
conducted experiments on the speed of light from 1877 until his death
in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s. The effective velocity of light in various transparent substances containing ordinary matter,
is less than in vacuum. For example the speed of light in water is
about 3/4 of that in vacuum. However, the slowing process in matter is
thought to result not from actual slowing of particles of light, but
rather from their absorption and re-emission from charged particles in
matter. As an extreme example of the nature of light-slowing in matter, two
independent teams of physicists were able to bring light to a "complete
standstill" by passing it through a Bose-Einstein Condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.
However, the popular description of light being "stopped" in these
experiments refers only to light being stored in the excited states of
atoms, then re-emitted at an arbitrary later time, as stimulated by a
second laser pulse. During the time it had "stopped" it had ceased to be
light.
[ Ultraviolet Light (this is what causes sunburn) ]
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| Electric arcs produce UV light, and arc welders must wear eye protection to prevent welder's flash. |
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, that is, in the range 10 nm to 400 nm, corresponding to photon energies from 3 eV
to 124 eV. It is so-named because the spectrum consists of
electromagnetic waves with frequencies higher than those that humans
identify as the colour violet. These frequencies are invisible to
humans, but visible to a number of insects and birds. UV light is found in sunlight (where it constitutes about 10% of the energy in vacuum) and is emitted by electric arcs and specialized lights such as black lights. It can cause chemical reactions, and causes many substances to glow or fluoresce. Most ultraviolet is classified as non-ionizing radiation. The higher energies of the ultraviolet spectrum from wavelengths about 10 nm to 120 nm ('extreme' ultraviolet) are ionizing, but this type of ultraviolet in sunlight is blocked by normal dioxygen in air, and does not reach the ground.
However, the entire spectrum of ultraviolet radiation has some of the
biological features of ionizing radiation, in doing far more damage to
many molecules in biological systems than is accounted for by simple
heating effects (an example is sunburn).
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| False-color image of the Sun's corona as seen in extreme ultraviolet (at 17.1 nm) by the Extreme ultraviolet Imaging Telescope |
These properties derive from the ultraviolet photon's power to alter chemical bonds in molecules, even without having enough energy to ionize atoms. Although ultraviolet radiation is invisible to the human eye, most people are aware of the effects of UV on the skin, called suntan and sunburn.
In addition to short wave UV blocked by oxygen, a great deal (>97%)
of mid-range ultraviolet (almost all UV above 280 nm and most above
315 nm) is blocked by the ozone layer,
and like ionizing short wave UV, would cause much damage to living
organisms if it penetrated the atmosphere. After atmospheric filtering,
only about 3% of the total energy of sunlight at the zenith is
ultraviolet,
and this fraction decreases at other sun angles. Much of it is
near-ultraviolet that does not cause sunburn. An even smaller fraction
of ultraviolet that reaches the ground is responsible for sunburn and
also the formation of vitamin D
(peak production occurring between 295 and 297 nm) in all organisms
that make this vitamin (including humans). The UV spectrum thus has many
effects, both beneficial and damaging, to human health.
[ X-rays ]
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| X-rays are part of the electromagnetic spectrum, with wavelength shorter than visible light. Different applications use different parts of the X-ray spectrum. |
X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. They are shorter in wavelength than UV rays and longer than gamma rays. In many languages, X-radiation is called Rontgen radiation, after Wilhelm Röntgen, who is usually credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation. Correct spelling of X-ray(s) in the English language includes the variants x-ray(s) and X ray(s). X-rays with photon energies above 5-10 keV (below 0.2-0.1 nm
wavelength), are called hard X-rays, while those with lower energy are
called soft X-rays. Due to their penetrating ability hard X-rays are widely used to image the inside of objects e.g. in medical radiography and airport security. As a result, the term X-ray is metonymically used to refer to a radiographic
image produced using this method, in addition to the method itself.
Since the wavelength of hard X-rays are similar to the size of atoms
they are also useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air and the attenuation length of 600 eV (~2 nm) X-rays in water is less than 1 micrometer.
The distinction between X-rays and gamma rays is somewhat arbitrary.
The most frequent method of distinguishing between X- and gamma
radiation is the basis of wavelength, with radiation shorter than some
arbitrary wavelength, such as 10−11 m, defined as gamma rays. The electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength than the radiation emitted by radioactive nuclei.
Historically, therefore, an alternative means of distinguishing between
the two types of radiation has been by their origin: X-rays are emitted
by electrons outside the nucleus, while gamma rays are emitted by the nucleus. There is overlap between the wavelength bands of photons emitted by
electrons outside the nucleus, and photons emitted by the nucleus. Like
all electromagnetic radiation, the properties of X-rays (or gamma rays)
depend only on their wavelength and polarization (or, in a polychromatic
beam, the distributions of wavelength and polarization).
[ Short waves ]
[ Microwaves, like in microwave oven ]
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| The atmospheric attenuation of microwaves in dry air with a precipitable water vapor level of 0.001 mm. The downward spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly. The right half of this graph includes the lower ranges of infrared by some standards |
Microwaves are radio waves with wavelengths ranging from as long as one meter to as short as one millimetre, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz. This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).
Apparatus and techniques may be described qualitatively as
"microwave" when the wavelengths of signals are roughly the same as the
dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower-frequency radio waves. Instead, distributed circuit elements and transmission-line theory are more useful methods for design and analysis. Open-wire and coaxial transmission lines give way to waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization, scattering, diffraction, and atmospheric absorption usually associated with visible light are of practical significance in the study of microwave propagation. The same equations of electromagnetic theory apply at all frequencies.
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| A microwave telecommunications tower |
The prefix "micro-" in "microwave" is not meant to suggest a
wavelength in the micrometer range. It indicates that microwaves are
"small" compared to waves used in typical radio broadcasting, in that
they have shorter wavelengths. The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study.
Microwave technology is extensively used for point-to-point telecommunications
(i.e., non broadcast uses). Microwaves are especially suitable for this
use since they are more easily focused into narrow beams than radio
waves, and also their comparitively higher frequencies allow broad
bandwidth and high data flow. Microwaves are the principal means by
which data, TV, and telephone communications are transmitted between
ground stations and to and from satellites. Microwaves are also employed
in microwave ovens and in radar technology. At about 20 GHz, decreasing microwave transmission through air is seen, due at lower frequencies from absorption
from water and at higher frequencies from oxygen. A spectral band
structure causes fluctuations in this behavior (see graph at right).
Above 300 GHz, the absorption of microwave electromagnetic radiation by
Earth's atmosphere is so great that it is in effect opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes such as potassium-40, and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin, are lightning strikes and terrestrial gamma-ray flashes,
which produce high energy emissions from natural high-energy voltages.
Gamma rays are produced by a number of astronomical processes in which
very high-energy electrons are produced. Such electrons produce
secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation.
A large fraction of such astronomical gamma rays are screened by
Earth's atmosphere and must be detected by spacecraft. Notable
artificial sources of gamma rays include fission such as occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.
[ Gamma rays ]
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency and therefore high energy. Gamma rays are ionizing radiation and are thus biologically hazardous. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay), but are also created by other processes. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium during its gamma decay. Villard's radiation was named "gamma rays" by Ernest Rutherford in 1903.
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| Gamma-ray image of a truck with two stowaways taken with a VACIS (vehicle and container imaging system) |
Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes, and secondary radiation from atmospheric interactions with cosmic ray particles. Rare terrestrial natural sources produce gamma rays that are not of a nuclear origin, such as lightning strikes and terrestrial gamma-ray flashes.
Gamma rays are produced by a number of astronomical processes in which
very high-energy electrons are produced, that in turn cause secondary
gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth's atmosphere and must be detected by spacecraft.
Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom).
However, this is not a hard and fast definition, but rather only a
rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV.
In astronomy, gamma rays are defined by their energy, and no production
process need be specified. The energies of gamma rays from astronomical
sources range over 10 TeV, at a level far too large to result from
radioactive decay. A notable example is extremely powerful bursts of
high-energy radiation normally referred to as long duration gamma-ray bursts,
which produce gamma rays by a mechanism not compatible with radioactive
decay. These bursts of gamma rays, thought to be due to the collapse of
stars called hypernovas, are the most powerful events so far discovered in the cosmos.
Waves
Any wave is essentially just a way of shifting energy from one place to another - whether the fairly obvious transfer of energy in waves on the sea or in the much more difficult-to-imagine waves in light. In waves on water, the energy is transferred by the movement of water molecules. But a particular water molecule doesn't travel all the way across the Atlantic - or even all the way across a pond. Depending on the depth of the water, water molecules follow a roughly circular path. As they move up to the top of the circle, the wave builds to a crest; as they move down again, you get a trough. The energy is transferred by relatively small local movements in the environment. With water waves it is fairly easy to draw diagrams to show this happening with real molecules. With light it is more difficult. The energy in light travels because of local fluctuating changes in electrical and magnetic fields - hence "electromagnetic" radiation.
All these waves do different things (for example, light waves make things visible to the human eye, while heat waves make molecules move and warm up, and x rays can pass through a person and land on film, allowing us to take a picture inside someone's body) but they have some things in common. They all travel in waves, like the waves at a beach or like sound waves, and also are made of tiny particles. Scientists are unsure of exactly how the waves and the particles relate to each other. The fact that electromagnetic radiation travels in waves lets us measure the different kind by wavelength or how long the waves are. That is one way we can tell the kinds of radiation apart from each other.
Although all kinds of electromagnetic radiation are released from the Sun, our atmosphere stops some kinds from getting to us. For example, the ozone layer stops a lot of harmful ultraviolet radiation from getting to us, and that's why people are so concerned about the hole in it. We humans have learned uses for a lot of different kinds of electromagnetic radiation and have learned how to make it using other kinds of energy when we need to. DS1 would not be able to communicate with Earth, for example, if it could not produce radio waves.
All these waves do different things (for example, light waves make things visible to the human eye, while heat waves make molecules move and warm up, and x rays can pass through a person and land on film, allowing us to take a picture inside someone's body) but they have some things in common. They all travel in waves, like the waves at a beach or like sound waves, and also are made of tiny particles. Scientists are unsure of exactly how the waves and the particles relate to each other. The fact that electromagnetic radiation travels in waves lets us measure the different kind by wavelength or how long the waves are. That is one way we can tell the kinds of radiation apart from each other.
Although all kinds of electromagnetic radiation are released from the Sun, our atmosphere stops some kinds from getting to us. For example, the ozone layer stops a lot of harmful ultraviolet radiation from getting to us, and that's why people are so concerned about the hole in it. We humans have learned uses for a lot of different kinds of electromagnetic radiation and have learned how to make it using other kinds of energy when we need to. DS1 would not be able to communicate with Earth, for example, if it could not produce radio waves.
An electromagnetic wave has a few important properties:
- Speed: how fast is each ripple moving?
- Frequency: if you point at the water with your finger, how many ripples pass by your finger every second?
- Wavelength: the distance between two adjacent ripples.
The speed is easy. It turns out that all electromagnetic waves have the same speed, which scientists represent with the letter c. This speed, the speed of light, is equal to 670 million miles per hour.
The frequency can be any number. It is measured in Hertz,
which means "One ripple per second." If two ripples pass by your finger
every second, that's 2 Hertz. Most electromagnetic radiation has
frequencies much larger than 1 Hertz. So we use larger units to measure
the frequency:
| kHz | kiloHertz, one thousand Hertz. (1,000) |
| MHz | MegaHertz, one million Hertz. (1,000,000) |
| GHz | GigaHertz, one billion Hertz. (1,000,000,000) |
For numbers that are too large to express this way, we use
"scientific notation", which is just a way of saying how many zeroes are
after a number. For instance, 1 MegaHertz could be written 1E6 Hertz,
meaning 1 with 6 zeroes after it. 2 GigaHertz is 2E9 Hertz. And
8,000,000,000,000 Hertz is 8E12 Hertz.
Here are some examples of frequencies of electromagnetic waves: *
| long-wave AM radio | 200 kHz |
| medium-wave AM radio | 1 MHz |
| short-wave AM radio | 10 MHz |
| FM Radio waves | 100 MHz |
| Microwaves in a microwave oven | 2.4 GHz |
| Infrared light | 3E12 Hz |
| Red light | 4E14 Hz |
| Green light | 6E14 Hz |
| Blue light | 7E14 Hz |
| Ultraviolet light | 1E15 Hz |
| X-rays | 3E18 Hz |
| Gamma rays | 3E20 Hz |
"Frequency" means the same thing for electromagnetic waves that it
does for sound waves. In a sound wave, the frequency is the number of
sound ripples that pass by in one second. For instance, the "A 440"
note, which orchestras use to tune up, has a frequency of 440 Hz. The
difference is that in an electromagnetic wave, the ripples are made of
electric and magnetic fields, whereas in a sound wave, the ripples are
made of moving air. Both Astropulse and the original SETI@home use frequencies around 1,420 MHz, ranging from 1,417.5 MHz from 1422.5 MHz.
* In fact, each of these waves actually has a range of frequencies. For instance, FM radio waves actually range from 87.5 to 108.0 MHz.
