Monday 16 July 2012

SAINS PPISMP


Electromagnetic radiation
From Wikipedia, the free encyclopedia


Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a form of energy exhibiting wave like behavior as it travels through space. EMR has both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation.
Electromagnetic radiation is classified according to the frequency of its wave. In order of increasing frequency and decreasing wavelength, these are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays (see Electromagnetic spectrum). The eyes of various organisms sense a small and somewhat variable window of frequencies called the visible spectrum. The photon is the quantum of the electromagnetic interaction and the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force.
EM radiation carries energy and momentum that may be imparted to matter with which it interacts.
Physics
Theory

Shows three electromagnetic modes (blue, green and red) with a distance scale in micrometres along the x-axis.
Main article: Maxwell's equations
James Clerk Maxwell first formally-postulated Electromagnetic waves. These were subsequently confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields, and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave.
According to Maxwell's equations, a time-varying electric field generates a time-varying magnetic field and vice versa. Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form a propagating electromagnetic wave.
A quantum theory of the interaction between electromagnetic radiation and matter such as electrons is described by the theory of quantum electrodynamics.
Properties

Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating from right to left. The electric field is in a vertical plane and the magnetic field in a horizontal plane.
The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey the properties of superposition. Thus, a field due to any particular particle or time-varying electric or magnetic field contributes to the fields present in the same space due to other causes. Further, as they are vector fields, all magnetic and electric field vectors add together according to vector addition. For example, in optics two or more coherent lightwaves may interact and by constructive or destructive interference yield a resultant irradiance deviating from the sum of the component irradiances of the individual lightwaves.
Since light is an oscillation it is not affected by travelling through static electric or magnetic fields in a linear medium such as a vacuum. However in nonlinear media, such as some crystals, interactions can occur between light and static electric and magnetic fields — these interactions include the Faraday effect and the Kerr effect.
In refraction, a wave crossing from one medium to another of different density alters its speed and direction upon entering the new medium. The ratio of the refractive indices of the media determines the degree of refraction, and is summarized by Snell's law. Light disperses into a visible spectrum as light is shone through a prism because of the wavelength dependent refractive index of the prism material (Dispersion).
EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). Both wave and particle characteristics have been confirmed in a large number of experiments. Wave characteristics are more apparent when EM radiation is measured over relatively large timescales and over large distances while particle characteristics are more evident when measuring small timescales and distances. For example, when electromagnetic radiation is absorbed by matter, particle-like properties will be more obvious when the average number of photons in the cube of the relevant wavelength is much smaller than 1. Upon absorption of light, it is not too difficult to experimentally observe non-uniform deposition of energy. Strictly speaking, however, this alone is not evidence of "particulate" behavior of light, rather it reflects the quantum nature of matter.
There are experiments in which the wave and particle natures of electromagnetic waves appear in the same experiment, such as the self-interference of a single photon. True single-photon experiments (in a quantum optical sense) can be done today in undergraduate-level labs.[2] When a single photon is sent through an interferometer, it passes through both paths, interfering with itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once.
 Wave model

White light being separated into its components.
Electromagnetic radiation is a transverse wave meaning that the oscillations of the waves are perpendicular to the direction of energy transfer and travel. An important aspect of the nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, where one hertz is equal to one oscillation per second. Light usually has a spectrum of frequencies which sum together to form the resultant wave. Different frequencies undergo different angles of refraction.
A wave consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called the wavelength. Waves of the electromagnetic spectrum vary in size, from very long radio waves the size of buildings to very short gamma rays smaller than atom nuclei. Frequency is inversely proportional to wavelength, according to the equation:
where v is the speed of the wave (c in a vacuum, or less in other media), f is the frequency and λ is the wavelength. As waves cross boundaries between different media, their speeds change but their frequencies remain constant.
Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference.
The energy in electromagnetic waves is sometimes called radiant energy.
Particle model
Because energy of an EM wave is quantized, in the particle model of EM radiation, a wave consists of discrete packets of energy, or quanta, called photons.[3] The frequency of the wave is proportional to the particle's energy. Because photons are emitted and absorbed by charged particles, they act as transporters of energy. The energy per photon can be calculated from the Planck–Einstein equation:[4]
where E is the energy, h is Planck's constant, and f is frequency. It is commonly expressed in the unit of electronvolt (eV). This photon-energy expression is a particular case of the energy levels of the more general electromagnetic oscillator, whose average energy, which is used to obtain Planck's radiation law, can be shown to differ sharply from that predicted by the equipartition principle at low temperature, thereby establishes a failure of equipartition due to quantum effects at low temperature.[5]
As a photon is absorbed by an atom, it excites the atom, elevating an electron to a higher energy level. If the energy is great enough, so that the electron jumps to a high enough energy level, it may escape the positive pull of the nucleus and be liberated from the atom in a process called photoionisation. Conversely, an electron that descends to a lower energy level in an atom emits a photon of light equal to the energy difference. Since the energy levels of electrons in atoms are discrete, each element emits and absorbs its own characteristic frequencies.
Together, these effects explain the emission and absorption spectra of light. The dark bands in the absorption spectrum are due to the atoms in the intervening medium absorbing different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, dark bands in the light emitted by a distant star are due to the atoms in the star's atmosphere. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons. This is manifested in the emission spectrum of nebulae. Today, scientists use this phenomenon to observe what elements a certain star is composed of. It is also used in the determination of the distance of a star, using the red shift.
[edit] Speed of propagation
Main article: Speed of light
Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. At the quantum level, electromagnetic radiation is produced when the wavepacket of a charged particle oscillates or otherwise accelerates. Charged particles in a stationary state do not move, but a superposition of such states may result in oscillation, which is responsible for the phenomenon of radiative transition between quantum states of a charged particle.
Depending on the circumstances, electromagnetic radiation may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10−34 J·s is Planck's constant, and ν is the frequency of the wave.
One rule is always obeyed regardless of the circumstances: EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)
In a medium (other than vacuum), velocity factor or refractive index are considered, depending on frequency and application. Both of these are ratios of the speed in a medium to speed in a vacuum.
[edit] Thermal radiation and electromagnetic radiation as a form of heat
Main article: Thermal radiation
The basic structure of matter involves charged particles bound together in many different ways. When electromagnetic radiation is incident on matter, it causes the charged particles to oscillate and gain energy. The ultimate fate of this energy depends on the situation. It could be immediately re-radiated and appear as scattered, reflected, or transmitted radiation. It may also get dissipated into other microscopic motions within the matter, coming to thermal equilibrium and manifesting itself as thermal energy in the material. With a few exceptions such as fluorescence, harmonic generation, photochemical reactions and the photovoltaic effect, absorbed electromagnetic radiation simply deposits its energy by heating the material. This happens both for infrared and non-infrared radiation. Intense radio waves can thermally burn living tissue and can cook food. In addition to infrared lasers, sufficiently intense visible and ultraviolet lasers can also easily set paper afire. Ionizing electromagnetic radiation can create high-speed electrons in a material and break chemical bonds, but after these electrons collide many times with other atoms in the material eventually most of the energy gets downgraded to thermal energy, this whole process happening in a tiny fraction of a second. That infrared radiation is a form of heat and other electromagnetic radiation is not, is a widespread misconception in physics. Any electromagnetic radiation can heat a material when it is absorbed.
The inverse or time-reversed process of absorption is responsible for thermal radiation. Much of the thermal energy in matter consists of random motion of charged particles, and this energy can be radiated away from the matter. The resulting radiation may subsequently be absorbed by another piece of matter, with the deposited energy heating the material. Radiation is an important mechanism of heat transfer.
The electromagnetic radiation in an opaque cavity at thermal equilibrium is effectively a form of thermal energy, having maximum radiation entropy. The thermodynamic potentials of electromagnetic radiation can be well-defined as for matter. Thermal radiation in a cavity has energy density (see Planck's Law) of
Differentiating the above with respect to temperature, we may say that the electromagnetic radiation field has an effective volumetric heat capacity given by
Electromagnetic spectrum

Electromagnetic spectrum with light highlighted
Legend:
γ =
Gamma rays
HX = Hard
X-rays
SX = Soft X-Rays
EUV = Extreme
ultraviolet
NUV = Near ultraviolet
Visible light
NIR = Near
infrared
MIR = Moderate infrared
FIR = Far infrared

Radio waves:
EHF =
Extremely high frequency (Microwaves)
SHF =
Super high frequency (Microwaves)
UHF =
Ultrahigh frequency
VHF =
Very high frequency
HF =
High frequency
MF =
Medium frequency
LF =
Low frequency
VLF =
Very low frequency
VF =
Voice frequency
ULF =
Ultra low frequency
SLF =
Super low frequency
ELF =
Extremely low frequency
Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. Arbitrary electromagnetic waves can always be expressed by Fourier analysis in terms of sinusoidal monochromatic waves which can be classified into these regions of the spectrum.
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. 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. It is widely used in astrophysics. For example, hydrogen atoms emit radio waves of wavelength 21.12 cm.
Soundwaves are not electromagnetic radiation. At the lower end of the electromagnetic spectrum, about 20 Hz to about 20 kHz, are frequencies that might be considered in the audio range, however, electromagnetic waves cannot be directly perceived by human ears. Sound waves are the oscillating compression of molecules. To be heard, electromagnetic radiation must be converted to air pressure waves, or if the ear is submerged, water pressure waves.
 Light
Main article: Light
EM radiation with a wavelength between approximately 400 nm and 700 nm is directly detected by the human eye and perceived as visible light. Other wavelengths, especially nearby infrared (longer than 700 nm) and ultraviolet (shorter than 400 nm) are also sometimes referred to as light, especially when visibility to humans is not relevant.
If radiation having a frequency in the visible region of the EM spectrum reflects off of an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-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 our technology can also manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data that can be translated into sound or an image. To be meaningful both transmitter and receiver must use some agreed-upon encoding system - especially so if the transmission is digital as opposed to the analog nature of the waves.
[edit] Radio waves
Main article: Radio waves
Radio waves can be made to carry information by varying a combination of the amplitude, frequency and phase of the wave within a frequency band.
When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens. Radio waves are not ionizing radiation, as the energy per photon is too small.
Derivation
Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. Beginning with Maxwell's equations in free space:
where
is a vector differential operator (see Del).
One solution,
,
is trivial.
To see the more interesting one, we utilize vector identities, which work for any vector, as follows:
To see how we can use this, take the curl of equation (2):
Evaluating the left hand side:
where we simplified the above by using equation (1).
Evaluate the right hand side:
Equations (6) and (7) are equal, so this results in a vector-valued differential equation for the electric field, namely
Applying a similar pattern results in similar differential equation for the magnetic field:
.
These differential equations are equivalent to the wave equation:
where
c0 is the speed of the wave in free space and
f describes a displacement
Or more simply:
where is d'Alembertian:
Notice that in the case of the electric and magnetic fields, the speed is:
Which, as it turns out, is the speed of light in vacuum. Maxwell's equations have unified the vacuum permittivity ε0, the vacuum permeability μ0, and the speed of light itself, c0. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism.
But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field.
Here is the constant amplitude, f is any second differentiable function, is a unit vector in the direction of propagation, and is a position vector. We observe that is a generic solution to the wave equation. In other words
,
for a generic wave traveling in the direction.
This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field?
The first of Maxwell's equations implies that electric field is orthogonal to the direction the wave propagates.
The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of .
Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes, E0 = c0B0, which can be seen immediately from the Poynting vector. The electric field, magnetic field, and direction of wave propagation are all orthogonal, and the wave propagates in the same direction as .
From the viewpoint of an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but this picture can be rotated with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation with respect to propagation direction is known as polarization. On a quantum level, it is described as photon polarization.
More general forms of the second order wave equations given above are available, allowing for both non-vacuum propagation media and sources. A great many competing derivations exist, all with varying levels of approximation and intended applications. One very general example is a form of the electric field equation,[6] which was factorized into a pair of explicitly directional wave equations, and then efficiently reduced into a single uni-directional wave equation by means of a simple slow-evolution approximation.
What are electromagnetic waves?
Electricity can be static, like what holds a balloon to the wall or makes your hair stand on end.



Magnetism can also be static like a refrigerator magnet. But when they change or move together, they make waves - electromagnetic waves.

Electromagnetic waves are formed when an electric field (shown as blue arrows) couples with a magnetic field (shown as red arrows). The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave. James Clerk Maxwell and Heinrich Hertz are two scientists who studied how electromagnetic waves are formed and how fast they travel.
Electromagnetic Waves have different wavelengths
When you listen to the radio, watch TV, or cook dinner in a microwave oven, you are using electromagnetic waves.

Radio waves, television waves, and microwaves are all types of electromagnetic waves. They differ from each other in wavelength. Wavelength is the distance between one wave crest to the next.
Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom.
Did you know that electromagnetic waves can not only be described by their wavelength, but also by their energy and frequency? All three of these things are related to each other mathematically. This means that it is correct to talk about the energy of an X-ray or the wavelength of a microwave or the frequency of a radio wave. The electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, X-rays, and gamma-rays.
Microwaves
Microwaves have wavelengths that can be measured in centimeters! The longer microwaves, those closer to a foot in length, are the waves which heat our food in a microwave oven.

Microwaves are good for transmitting information from one place to another because microwave energy can penetrate haze, light rain and snow, clouds, and smoke.
Shorter microwaves are used in remote sensing. These microwaves are used for radar like the doppler radar used in weather forecasts. Microwaves, used for radar, are just a few inches long.
This microwave tower can transmit information like telephone calls and computer data from one city to another.

How do we "see" using Microwaves?
Radar is an acronym for "radio detection and ranging". Radar was developed to detect objects and determine their range (or position) by transmitting short bursts of microwaves. The strength and origin of "echoes" received from objects that were hit by the microwaves is then recorded.
Because radar senses electromagnetic waves that are a reflection of an active transmission, radar is considered an active remote sensing system. Passive remote sensing refers to the sensing of electromagnetic waves which did not originate from the satellite or sensor itself. The sensor is just a passive observer.

What do Microwaves show us?
Because microwaves can penetrate haze, light rain and snow, clouds and smoke, these waves are good for viewing the Earth from space.
The ERS-1 satellite sends out wavelengths about 5.7 cm long (C-band). This image shows sea ice breaking off the shores of Alaska.

The JERS satellite uses wavelengths about 20 cm in length (L-band). This is an image of the Amazon River in Brazil.

This is a radar image acquired from the Space Shuttle. It also used a wavelength in the L-band of the microwave spectrum. Here we see a computer enhanced radar image of some mountains on the edge of Salt Lake City, Utah.
In the 1960's a startling discovery was made quite by accident. A pair of scientists at Bell Laboratories detected background noise using a special low noise antenna. The strange thing about the noise was that it was coming from every direction and did not seem to vary in intensity much at all. If this static were from something on our world, like radio transmissions from a nearby airport control tower, it would only come from one direction, not everywhere. The scientists soon realized they had discovered the cosmic microwave background radiation. This radiation, which fills the entire Universe, is believed to be a clue to it's beginning, something known as the Big Bang.
The image above is a Cosmic Background Explorer (COBE) image of the cosmic microwave background, the pink and blue colors showing the tiny fluctuations in it.
Did you know that if you had a sensitive microwave telescope in your house that you would detect a faint signal leaking out of your microwave oven, and from various other man-made sources, but also a faint signal coming from all directions that you pointed it? This is the Cosmic Microwave Background!
Radio Waves
Radio waves have the longest wavelengths in the electromagnetic spectrum. These waves can be longer than a football field or as short as a football. Radio waves do more than just bring music to your radio. They also carry signals for your television and cellular phones.

The antennae on your television set receive the signal, in the form of electromagnetic waves, that is broadcasted from the television station. It is displayed on your television screen.
Cable companies have antennae or dishes which receive waves broadcasted from your local TV stations. The signal is then sent through a cable to your house.
Why are car antennae about the same size as TV antennae?

Cellular phones also use radio waves to transmit information. These waves are much smaller that TV and FM radio waves.
Why are antennae on cell phones smaller than antennae on your radio?

How do we "see" using Radio Waves?
Objects in space, such as planets and comets, giant clouds of gas and dust, and stars and galaxies, emit light at many different wavelengths. Some of the light they emit has very large wavelengths - sometimes as long as a mile!. These long waves are in the radio region of the electromagnetic spectrum.
Because radio waves are larger than optical waves, radio telescopes work differently than telescopes that we use for visible > light (optical telescopes). Radio telescopes are dishes made out of conducting metal that reflect radio waves to a focus point. Because the wavelengths of radio light are so large, a radio telescope must be physically larger than an optical telescope to be able to make images of comparable clarity. For example, the Parkes radio telescope, which has a dish 64 meters wide, cannot give us any clearer an image than a small backyard telescope!
In order to make better and more clear (or higher resolution) radio images, radio astronomers often combine several smaller telescopes, or receiving dishes, into an array. Together, the dishes can act as one large telescope whose size equals the total area occupied by the array.

The Very Large Array (VLA) is one of the world's premier astronomical radio observatories. The VLA consists of 27 antennas arranged in a huge "Y" pattern up to 36 km (22 miles) across -- roughly one and a half times the size of Washington, DC.
The VLA, located in New Mexico, is an interferometer; this means that it operates by multiplying the data from each pair of telescopes together to form interference patterns. The structure of those interference patterns, and how they change with time as the earth rotates, reflect the structure of radio sources in the sky.

What do Radio Waves show us?
The above image shows the Carbon Monoxide (CO) gases in our Milky Way galaxy.
Many astronomical objects emit radio waves, but that fact wasn't discovered until 1932. Since then, astronomers have developed sophisticated systems that allow them to make pictures from the radio waves emitted by astronomical objects.
Radio telescopes look toward the heavens at planets and comets, giant clouds of gas and dust, and stars and galaxies. By studying the radio waves originating from these sources, astronomers can learn about their composition, structure, and motion. Radio astronomy has the advantage that sunlight, clouds, and rain do not affect observations.

Gamma-rays
Gamma-rays have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum. These waves are generated by radioactive atoms and in nuclear explosions. Gamma-rays can kill living cells, a fact which medicine uses to its advantage, using gamma-rays to kill cancerous cells.
Gamma-rays travel to us across vast distances of the universe, only to be absorbed by the Earth's atmosphere. Different wavelengths of light penetrate the Earth's atmosphere to different depths. Instruments aboard high-altitude balloons and satellites like the Compton Observatory provide our only view of the gamma-ray sky.
Gamma-rays are the most energetic form of light and are produced by the hottest regions of the universe. They are also produced by such violent events as supernova explosions or the destruction of atoms, and by less dramatic events, such as the decay of radioactive material in space. Things like supernova explosions (the way massive stars die), neutron stars and pulsars, and black holes are all sources of celestial gamma-rays.

How do we "see" using gamma-ray light?
Gamma-ray astronomy did not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope, carried into orbit on the Explorer XI satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons!
Unlike optical light and X-rays, gamma rays cannot be captured and reflected in mirrors. The high-energy photons would pass right through such a device. Gamma-ray telescopes use a process called Compton scattering, where a gamma-ray strikes an electron and loses energy, similar to a cue ball striking an eight ball.
This image shows the CGRO satellite being deployed from the Space Shuttle orbiter. This picture was taken from an orbiter window. The two round protrusions are one of CGRO's instruments, called "EGRET".

What do gamma-rays show us?
If you could see gamma-rays, the night sky would look strange and unfamiliar.
The gamma-ray moon just looks like a round blob - lunar features are not visible. In high-energy gamma rays, the Moon is actually brighter than the quiet Sun. This image was taken by EGRET.
Credit: D.J. Thompson, D.L. Bertsch (NASA/GSFC),
D.J. Morris (UNH), R. Mukherjee (NASA/GSFC/USRA)

The familiar sights of constantly shining stars and galaxies would be replaced by something ever-changing. Your gamma-ray vision would peer into the hearts of solar flares, supernovae, neutron stars, black holes, and active galaxies. Gamma-ray astronomy presents unique opportunities to explore these exotic objects. By exploring the universe at these high energies, scientists can search for new physics, testing theories and performing experiments which are not possible in earth-bound laboratories.
If you could see gamma-rays, these two spinning neutron stars or pulsars would be among the brightest objects in the sky. This computer processed image shows the Crab Nebula pulsar (below and right of center) and the Geminga pulsar (above and left of center) in the "light" of gamma-rays.
The Crab nebula, shown also in the visible light image, was created by a supernova that brightened the night sky in 1054 A.D. In 1967, astronomers detected the remnant core of that star; a rapidly rotating, magnetic pulsar flashing every 0.33 second in radio waves.
Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s. Detectors on board the Vela satellite series, originally military satellites, began to record bursts of gamma-rays -- not from Earth, but from deep space!
Today, these gamma-ray bursts, which happen at least once a day, are seen to last for fractions of a second to minutes, popping off like cosmic flashbulbs from unexpected directions, flickering, and then fading after briefly dominating the gamma-ray sky.
Gamma-ray bursts can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime! So far, it appears that all of the bursts we have observed have come from outside the Milky Way Galaxy. Scientists believe that a gamma-ray burst will occur once every few million years here in the Milky Way, and in fact may occur once every several hundred million years within a few thousand light-years of Earth.
Studied for over 25 years now with instruments on board a variety of satellites and space probes, including Soviet Venera spacecraft and the Pioneer Venus Orbiter, the sources of these enigmatic high-energy flashes remain a mystery.
By solving the mystery of gamma-ray bursts, scientists hope to gain further knowledge of the origins of the Universe, the rate at which the Universe is expanding, and the size of the Universe.
X-rays
As the wavelengths of light decrease, they increase in energy. X-rays have smaller wavelengths and therefore higher energy than ultraviolet waves. We usually talk about X-rays in terms of their energy rather than wavelength. This is partially because X-rays have very small wavelengths. It is also because X-ray light tends to act more like a particle than a wave. X-ray detectors collect actual photons of X-ray light - which is very different from the radio telescopes that have large dishes designed to focus radio waves!
X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident when experimenting with vacuum tubes.
A week later, he took an X-ray photograph of his wife's hand which clearly revealed her wedding ring and her bones. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Roentgen called it "X" to indicate it was an unknown type of radiation. The name stuck, although (over Roentgen's objections), many of his colleagues suggested calling them Roentgen rays. They are still occasionally referred to as Roentgen rays in German-speaking countries.
The Earth's atmosphere is thick enough that virtually no X-rays are able to penetrate from outer space all the way to the Earth's surface. This is good for us but also bad for astronomy - we have to put X-ray telescopes and detectors on satellites! We cannot do X-ray astronomy from the ground.

How do we "see" using X-ray light?
What would it be like to see X-rays? Well, we wouldn't be able to see through people's clothes, no matter what the ads for X-ray glasses tell us! If we could see X-rays, we could see things that either emit X-rays or halt their transmission. Our eyes would be like the X-ray film used in hospitals or dentist's offices. X-ray film "sees" X-rays, like the ones that travel through your skin. It also sees shadows left by things that the X-rays can't travel through (like bones or metal).
When you get an X-ray taken at a hospital, X-ray sensitive film is put on one side of your body, and X-rays are shot through you. At a dentist, the film is put inside your mouth, on one side of your teeth, and X-rays are shot through your jaw, just like in this picture. It doesn't hurt at all - you can't feel X-rays.
Because your bones and teeth are dense and absorb more X-rays then your skin does, silhouettes of your bones or teeth are left on the X-ray film while your skin appears transparent. Metal absorbs even more X-rays - can you see the filling in the image of the tooth?
When the Sun shines on us at a certain angle, our shadow is projected onto the ground. Similarly, when X-ray light shines on us, it goes through our skin, but allows shadows of our bones to be projected onto and captured by film.
This is an X-ray photo of a one year old girl. Can you see the shadow of what she swallowed?
We use satellites with X-ray detectors on them to do X-ray astronomy. In astronomy, things that emit X-rays (for example, black holes) are like the dentist's X-ray machine, and the detector on the satellite is like the X-ray film. X-ray detectors collect individual X-rays (photons of X-ray light) and things like the number of photons collected, the energy of the photons collected, or how fast the photons are detected, can tell us things about the object that is emitting them.
To the right is an image of a real X-ray detector. This instrument is called the Proportional Counter Array and it is on the Rossi X-ray Timing Explorer (RXTE) satellite. It looks very different from anything you might see at a dentist's office!

What does X-ray light show us?
Many things in space emit X-rays, among them are black holes, neutron stars, binary star systems, supernova remnants, stars, the Sun, and even some comets!
The Earth glows in many kinds of light, including the energetic X-ray band. Actually, the Earth itself does not glow - only aurora produced high in the Earth's atmosphere. These aurora are caused by charged particles from the Sun.
Credit: Polar, PIXIE, NASA
To the left is the first picture of the Earth in X-rays, taken in March, 1996 with the orbiting Polar satellite. The area of brightest X-ray emission is red. The energetic charged particles from the Sun that cause aurora also energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the Earth's ionosphere, causing the X-ray emission. These X-rays are not dangerous because they are absorbed by lower parts of the Earth's atmosphere. (The above caption and image are from the Astronomy Picture of the Day for December 30, 1996.)

Recently, we learned that even comets emit X-rays! This image of Comet Hyakutake was taken by an X-ray satellite called ROSAT, short for the Roentgen Satellite. (It was named after the discoverer of X-rays.)

The Sun also emits X-rays - here is what the Sun looked like in X-rays on April 27th, 2000. This image was taken by the Yokoh satellite.

Many things in deep space give off X-rays. Many stars are in binary star systems - which means that two stars orbit each other. When one of these stars is a black hole or a neutron star, material is pulled off the normal star. This materials spirals into the black hole or neutron star and heats up to very high temperatures. When something is heated to over a million degrees, it will give off X-rays!
The above image is an artist's conception of a binary star system - it shows the material being pulled off the red star by its invisible black hole companion and into an orbiting disk.
Credit: X-ray (NASA/CXC/SAO);
Optical (NASA/HST);
Radio: (CSIRO/ATNF/ATCA)
This image is special - it shows a supernova remnant - the remnant of a star that exploded in a nearby galaxy known as the Small Magellanic Cloud. The false-colors show what this supernova remnant looks like in X-rays (in blue), visible light (green) and radio (red).

Credit: NASA/CXC/SAO
This is the same supernova remnant but this image shows only X-ray emission.


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