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The Tyndall effect is an effect of light scattering by colloidal particles or particles in
suspension. It is named after the 19th century Irish scientist John Tyndall. It is similar to
Rayleigh scattering, in that the intensity of the scattered light depends on the fourth
power of the frequency, so blue light is scattered more strongly than red light. An
example in everyday life is the blue colour sometimes seen in the smoke emitted by
motor bikes. The phenomenon is best explained by Mie theory as the particle size is
much greater than the wavelength of light.
The basis for distinguishing between Tyndall and Rayleigh scattering is the intensity of
the phenomenon. Colloidal particles are much larger than atoms or molecules. It follows
from scattering theory that Tyndall scattering (by colloidal particles) is much more
intense than Rayleigh scattering (by atoms or molecules). Tyndall scattering can be used
to determine the size of colloidal particles. Tyndall scattering is also often used to
describe light scattering by macroscopic particles such as dust in the air. However, this
phenomenon is more like reflection, as the macroscopic particles become visible in the
process.
When measuring Raman spectra of solutions, ultrafiltration may be needed to minimize
interference by Tyndall scattering.
Prolonged and brilliantly coloured sunsets and twilights were frequently seen in London
in the summer of 1815 due to Tyndall scattering of sunlight by ash particles in the upper
atmosphere, produced by the earlier eruption of the volcano Tambora. Such a sunset may
have been the inspiration for the brilliant and evocative sunset in the painting The
Fighting Temeraire, though a similar effect could have been produced by Rayleigh
scattering with exceptionally high atmospheric pressure.
Porous glass filled with water, sample about 1 mm thick, made by phase separation in a
thermal gradient (high temperature on the right) of a sodium borosilicate glass, followed
by acid leaching. The colors are caused by the Tyndall effect.
Tyndall effect
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Flour suspended in water appears to be blue because blue light is scattered by the flour
particles more strongly than red light.
Porous glass filled with water, sample about 1 mm thick, made by phase separation in a
thermal gradient (high temperature on the right) of a sodium borosilicate glass, followed
by acid leaching. The colors are caused by the Tyndall effect.
The Tyndall effect is an effect of light scattering by colloidal particles or particles in
suspension. It is named after the 19th century Irish scientist John Tyndall. It is similar to
Rayleigh scattering, in that the intensity of the scattered light depends on the fourth
power of the frequency, so blue light is scattered more strongly than red light. An
example in everyday life is the blue colour sometimes seen in the smoke emitted by
motor bikes. The phenomenon is best explained by Mie theory as the particle size is
much greater than the wavelength of light.
The basis for distinguishing between Tyndall and Rayleigh scattering is the intensity of
the phenomenon. Colloidal particles are much larger than atoms or molecules. It follows
from scattering theory that Tyndall scattering (by colloidal particles) is much more
intense than Rayleigh scattering (by atoms or molecules). Tyndall scattering can be used
to determine the size of colloidal particles. Tyndall scattering is also often used to
describe light scattering by macroscopic particles such as dust in the air. However, this
phenomenon is more like reflection, as the macroscopic particles become visible in the
process.
When measuring Raman spectra of solutions, ultrafiltration may be needed to minimize
interference by Tyndall scattering.
Prolonged and brilliantly coloured sunsets and twilights were frequently seen in London
in the summer of 1815 due to Tyndall scattering of sunlight by ash particles in the upper
atmosphere, produced by the earlier eruption of the volcano Tambora. Such a sunset may
have been the inspiration for the brilliant and evocative sunset in the painting The
Fighting Temeraire, though a similar effect could have been produced by Rayleigh
scattering with exceptionally high atmospheric pressure
Rayleigh scattering
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This article is about the optical phenomenon. For the magnetic phenomenon, see
Rayleigh law.
Rayleigh scattering causes the blue hue of the daytime sky and the reddening of the sun at
sunset
Rayleigh scattering is more dramatic after sunset. This picture was taken about one hour
after sunset at 500m altitude, looking at the horizon where the sun had set.
Figure showing the more intense scattering of blue light by the atmosphere relative to red
light.
Rayleigh scattering (named after the English physicist Lord Rayleigh) is the elastic
scattering of light or other electromagnetic radiation by particles much smaller than the
wavelength of the light. It can occur when light travels in transparent solids and liquids,
but is most prominently seen in gases.
Rayleigh scattering of sunlight in clear atmosphere is the main reason why the sky is
blue: Rayleigh and cloud-mediated scattering contribute to diffuse light (direct light
being sunrays).
For scattering by particles similar to or larger than a wavelength, see Mie theory or
discrete dipole approximation (they apply to the Rayleigh regime as well).
Contents
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1 Small size parameter approximation
2 Rayleigh scattering from molecules
3 Reason for the blue color of the sky
4 See also
5 References
6 External links
[edit] Small size parameter approximation
The size of a scattering particle is parametrized by the ratio x of its characteristic
dimension r and wavelength λ:
Rayleigh scattering can be defined as scattering in the small size parameter regime x ≪ 1.
Scattering from larger spherical particles is explained by the Mie theory for an arbitrary
size parameter x. The Mie theory reduces to the Rayleigh approximation.
The amount of Rayleigh scattering that occurs for a beam of light is dependent upon the
size of the particles and the wavelength of the light. Specifically, the intensity of the
scattered light varies as the sixth power of the particle size and varies inversely with the
fourth power of the wavelength.
The intensity I of light scattered by a single small particle from a beam of unpolarized
light of wavelength λ and intensity I0 is given by:
where R is the distance to the particle, θ is the scattering angle, n is the refractive index of
the particle, and d is the diameter of the particle.
The angular distribution of Rayleigh scattering, governed by the (1 + cos2θ) term, is
symmetric about the plane normal to the incident direction of the light (i.e. about θ =
90°), and so the forward scatter equals the backwards scatter. Integrating over the sphere
surrounding the particle gives the Rayleigh scattering cross section
The Rayleigh scattering coefficient for a group of scattering particles is the number of
particles per unit volume N times the cross-section. As with all wave effects, for
incoherent scattering the scattered powers add arithmetically, while for coherent
scattering, such as if the particles are very near each other, the fields add arithmetically
and the sum must be squared to obtain the total scattered power.
[edit] Rayleigh scattering from molecules
A 5 mW green laser pointer is visible at night due to Rayleigh scattering and airborne
dust.
Rayleigh scattering from molecules is also possible. An individual molecule does not
have a well-defined refractive index and diameter. Instead, a molecule has a polarizability
α, which describes how much the electrical charges on the molecule will move in an
electric field. In this case, the Rayleigh scattering intensity for a single particle is given
by[1]
The amount of Rayleigh scattering from a single particle can also be expressed as a cross
section σ. For example, the major constituent of the atmosphere, nitrogen, has a Rayleigh
cross section of 5.1×10−31 m2 at a wavelength of 532 nm (green light).[2] This means that
at atmospheric pressure, about a fraction 10-5 of light will be scattered for every meter of
travel.
The strong wavelength dependence of the scattering (~λ−4) means that blue light is
scattered much more readily than red light. In the atmosphere, this results in blue
wavelengths being scattered to a greater extent than longer (red) wavelengths, and so one
sees blue light coming from all regions of the sky. Direct radiation (by definition) is
coming directly from the Sun. Rayleigh scattering is a good approximation to the manner
in which light scattering occurs within various media for which scattering particles have a
small size parameter.
Reason for the blue color of the sky
: Diffuse sky radiation
Rayleigh scattering is responsible for the blue color of the sky during the day. Rayleigh
scattering is inversely proportional to the fourth power of wavelength, which means that
the shorter wavelength of blue light will scatter more than the longer wavelengths of
green and red light. This gives the sky a blue appearance.
Conversely, looking toward the sun, the colors that were not scattered away -- the longer
wavelengths such as red and yellow light -- are visible. When the sun is near the horizon,
the volume of air through which sunlight must pass is significantly greater than when the
sun is high in the sky. Accordingly, the gradient from a red-yellow sun to the blue sky is
considerably wider at sunrise and sunset.
Rayleigh scattering primarily occurs through light's interaction with air molecules. Some
of the scattering can also be from aerosols of sulfate particles. For years following large
Plinian eruptions, the blue cast of the sky is notably brightened due to the persistent
sulfate load of the stratospheric eruptive gases. Another source of scattering is from
microscopic density fluctuations, resulting from the random motion of the air molecules.
A region of higher or lower density has a slightly different refractive index than the
surrounding medium, and therefore it acts like a short-lived particle that can scatter light
Raman scattering
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Raman scattering or the Raman effect (pronounced /ˈrɑːmən/) is the inelastic
scattering of a photon. Discovered by C.V. Raman in liquids[1] and by Grigory Landsberg
and Leonid Mandelstam in crystals[2]. The effect is sometimes called Smekal-Raman
effect due to the prediction by Adolf Smekal (de).[3]
When light is scattered from an atom or molecule, most photons are elastically scattered
(Rayleigh scattering), such that the scattered photons have the same energy (frequency)
and wavelength as the incident photons. However, a small fraction of the scattered light
(approximately 1 in 10 million photons) is scattered by an excitation, with the scattered
photons having a frequency different from, and usually lower than, the frequency of the
incident photons.[4] In a gas, Raman scattering can occur with a change in vibrational,
rotational or electronic energy of a molecule (see energy level). Chemists are concerned
primarily with the vibrational Raman effect.
In 1922, Indian physicist Chandrasekhara Venkata Raman published his work on the
"Molecular Diffraction of Light," the first of a series of investigations with his
collaborators which ultimately led to his discovery (on 28 February 1928) of the radiation
effect which bears his name. The Raman effect was first reported by C. V. Raman and K.
S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam, in 1928.
Raman received the Nobel Prize in 1930 for his work on the scattering of light. In 1998[5]
the Raman Effect was designated an ACS National Historical Chemical Landmark in
recognition of its significance as a tool for analyzing the composition of liquids, gases,
and solids.[6]
Contents
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1 Stokes and anti-Stokes scattering
2 Distinction with fluorescence
3 Selection rules
4 Stimulated scattering and amplification
5 Spectrum generation
6 Applications
7 See also
8 Notes
9 References
10 External links
[edit] Stokes and anti-Stokes scattering
The different possibilities of visual light scattering: Rayleigh scattering (no Raman
effect), Stokes scattering (molecule absorbs energy) and anti-Stokes scattering (molecule
loses energy)
There are two types of Raman scattering, Stokes scattering and anti-Stokes scattering.
The interaction of light with matter in a linear regime allows the absorption or
simultaneous emission of light precisely matching the difference in energy levels of the
interacting electrons.
The Raman effect corresponds, in perturbation theory, to the absorption and subsequent
emission of a photon via an intermediate electron state, having a virtual energy level (see
also: Feynman diagram). There are three possibilities:
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no energy exchange between the incident photons and the molecules (and hence
no Raman effect)
energy exchanges occur between the incident photons and the molecules. The
energy differences are equal to the differences of the vibrational and rotational
energy-levels of the molecule. In crystals only specific photons are allowed
(solutions of the wave equations which do not cancel themselves) by the periodic
structure, so Raman scattering can only appear at certain frequencies. In
amorphous materials like glasses, more photons are allowed and thereby the
discrete spectral lines become broad.
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molecule absorbs energy: Stokes scattering. The resulting photon of lower
energy generates a Stokes line on the red side of the incident spectrum.
molecule loses energy: anti-Stokes scattering. Incident photons are shifted
to the blue side of the spectrum, thus generating an anti-Stokes line.
These differences in energy are measured by subtracting the energy of the monoenergetic laser light from the energy of the scattered photons. The absolute value,
however, doesn't depend on the process (Stokes or anti-Stokes scattering), because only
the energy of the different vibrational levels is of importance. Therefore, the Raman
spectrum is symmetric relative to the Rayleigh band. In addition, the intensities of the
Raman bands are only dependent on the number of molecules occupying the different
vibrational states, when the process began. If the sample is in thermal equilibrium, the
relative numbers of molecules in states of different energy will be given by the
Boltzmann distribution:
where:
N0: number of atoms in the lower vibrational state
N1: number of atoms in the higher vibrational state
g0: degeneracy of the lower vibrational state (number of orbitals
of the same energy)
g1: degeneracy of the higher vibrational state
ΔEv: energy difference between these two vibrational states
k: Boltzmann's constant
T: temperature in kelvin
Thus lower energy states will have more molecules in them than will higher (excited)
energy states. Therefore, the Stokes spectrum will be more intense than the anti-Stokes
spectrum.
[edit] Distinction with fluorescence
The Raman effect differs from the process of fluorescence. For the latter, the incident
light is completely absorbed and the system is transferred to an excited state from which
it can go to various lower states only after a certain resonance lifetime. The result of both
processes is essentially the same: A photon with the frequency different from that of the
incident photon is produced and the molecule is brought to a higher or lower energy
level. But the major difference is that the Raman effect can take place for any frequency
of the incident light. In contrast to the fluorescence effect, the Raman effect is therefore
not a resonant effect. In practice, this means that a fluorescence peak is anchored at a
specific excitation frequency, whereas a Raman peak maintains a constant separation
from the excitation frequency. Another, related distinction is that Raman scattering is a
coherent process, whereas fluorescence is not. This means that the measured intensity is
the square of a coherent sum of scattering amplitudes. In practice, this means that
different paths to the excitation of the same mode may interfere, leading to Fano effects:
asymmetries in the shape of the scattering peaks.
[edit] Selection rules
The distortion of a molecule in an electric field, and therefore the vibrational Raman
cross section, is determined by its polarizability.
A Raman transition from one state to another, and therefore a Raman shift, can be
activated optically only in the presence of non-zero polarizability derivative with respect
to the normal coordinate (that is, the vibration or rotation):
Raman-active vibrations/rotations can be identified by using almost any textbook that
treats quantum mechanics or group theory for chemistry. Then, Raman-active modes can
be found for molecules or crystals that show symmetry by using the appropriate character
table for that symmetry group.
[edit] Stimulated scattering and amplification
Raman amplification can be obtained by using Stimulated Raman Scattering (SRS),
which actually is a combination of a Raman process with stimulated emission. It is
interesting for application in telecommunication fibers to amplify inside the standard
material with low noise for the amplification process. However, the process requires
significant power and thus imposes more stringent limits on the material. The
amplification band can be up to 100 nm broad, depending on the availability of allowed
photon states.
[edit] Spectrum generation
For high intensity CW (continuous wave) lasers, SRS can be used to produce broad
bandwidth spectra. This process can also be seen as a special case of four wave mixing,
where the frequencies of the two incident photons are equal and the emitted spectra are
found in two bands separated from the incident light by the phonon energies. The initial
Raman spectrum is built up with spontaneous emission and is amplified later on. At high
pumping levels in long fibers, higher order Raman spectra can be generated by using the
Raman spectrum as a new starting point, thereby building a chain of new spectra with
decreasing amplitude. The disadvantage of intrinsic noise due to the initial spontaneous
process can be overcome by seeding a spectrum at the beginning, or even using a
feedback loop like in a resonator to stabilize the process. Since this technology easily fits
into the fast evolving fiber laser field and there is demand for transversal coherent high
intensity light sources (i.e. broadband telecommunication, imaging applications), Raman
amplification and spectrum generation might be widely used in the near future.
[edit] Applications
Raman spectroscopy employs the Raman effect for materials analysis. The frequency of
light scattered from a molecule may be changed based on the structural characteristics of
the molecular bonds. A monochromatic light source (laser) is required for illumination,
and a spectrogram of the scattered light then shows the deviations caused by state
changes in the molecule.
Raman spectroscopy is used as a tool to detect high-frequency phonons, magnons and
electronic excitations.
Raman spectroscopy is also used in combustion diagnostics. Being a completely nonintrusive technique, it permits the detection of the major species and temperature
distribution inside combustors and in flames without any perturbation of the (mainly fluid
dynamic and reactive) processes examined.
Stimulated Raman transitions are also widely used for manipulating a trapped ion's
energy levels, and thus basis qubit states.
Raman amplification is used in optical amplifiers.
Raman lidar is used in Atmospheric Physics to study aerosol vertical distribution.
Brillouin scattering
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It has been suggested that this article or section be merged into Light_scattering.
(Discuss)
Brillouin scattering, named for Léon Brillouin, occurs when light in a medium (such as
water or a crystal) interacts with time dependent density variations and changes its energy
(frequency) and path. The density variations may be due to acoustic modes, such as
phonons, magnetic modes, such as magnons, or temperature gradients. As described in
classical physics, when the medium is compressed its index of refraction changes and the
light's path necessarily bends.
Contents
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1 Definition
2 Relationship to Raman scattering
3 Stimulated Brillouin scattering
4 Discovery
5 Fiber Optic Sensing
6 See also
7 References
8 External links
[edit] Definition
From a quantum point of view, Brillouin scattering is an interaction of light photons with
acoustic or vibrational quanta (phonons), with magnetic spin waves (magnons), or with
other low frequency quasiparticles interacting with light. The interaction consists of an
inelastic scattering process in which a phonon or magnon is either created (Stokes
process) or annihilated (anti-Stokes process). The energy of the scattered light is slightly
changed, that is decreased for a Stokes process and increased for an anti-Stokes process.
This shift, known as the Brillouin shift, is equal to the energy of the interacting phonon
and magnon and thus Brillouin scattering can be used to measure phonon and magnon
energies. The Brillouin shift is commonly measured by the use of a Brillouin
spectrometer based on a Fabry-Pérot interferometer.
[edit] Relationship to Raman scattering
Brillouin scattering is similar to Raman scattering in that both phenomena represent
inelastic scattering processes of light with quasiparticles. The difference lies in the
detected frequency shift range and type of information extracted from the sample.
Brillouin scattering denominates the scattering of photons from quasiparticles, while for
Raman scattering photons are scattered by interaction with vibrational and rotational
transitions in molecules. Therefore the two techniques provide very different information
about the sample: Raman spectroscopy is used to determine the chemical composition
and molecular structure, while Brillouin scattering measures properties on a larger scale –
such as the elastic behaviour. Experimentally, the frequency shifts in Brillouin scattering
are detected with an interferometer, while Raman setup can be based on either
interferometer or dispersive (grating) spectrometer.
[edit] Stimulated Brillouin scattering
For intense beams (e.g. laser light) travelling in a medium such as an optical fiber, the
variations in the electric field of the beam itself may produce acoustic vibrations in the
medium via electrostriction. The beam may undergo Brillouin scattering from these
vibrations, usually in opposite direction to the incoming beam, a phenomenon known as
stimulated Brillouin scattering (SBS). For liquids and gases, typical frequency shifts
are of the order of 1–10 GHz (wavelength shifts of ~1–10 pm for visible light).
Stimulated Brillouin scattering is one effect by which optical phase conjugation can take
place.
[edit] Discovery
The phenomenon of inelastic scattering of light due to acoustic phonons was first
described by Leon Brillouin (1889-1969) in 1922 and 4 years later in 1926 independently
by Leonid Mandelstam. In order to credit Mandelstam it is also denoted as BrillouinMandelstam scattering (BMS). Other commonly used names are Brillouin light scattering
(BLS) and Brillouin-Mandelstam light scattering (BMLS).
The process of stimulated Brillouin scattering (SBS) was first observed by Chiao et al. in
1964. The optical phase conjugation aspect of the SBS process was discovered by
Zel’dovich et al. in 1972.
[edit] Fiber Optic Sensing
Brillouin scattering can also be employed to sense mechanical strain and temperature in
optical fibers [1].
Compton scattering
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In physics, Compton scattering or
Compton scattering
the Compton effect is the decrease in
Feynman diagrams
energy (increase in wavelength) of an
X-ray or gamma ray photon, when it
s-channel
interacts with matter. Because of the
change in photon energy, it is an
u-channel
inelastic scattering process. Inverse
Compton scattering also exists,
where the photon gains energy
Light-matter interaction
(decreasing in wavelength) upon
interaction with matter. The amount
Low energy phenomena Photoelectric effect
the wavelength changes by is called
Mid-energy phenomena Compton scattering
the Compton shift. Although nuclear
compton scattering exists[1], Compton
High energy phenomena Pair production
scattering usually refers to the
interaction involving only the electrons of an atom. The Compton effect was observed by
Arthur Holly Compton in 1923 and further verified by his graduate student Y. H. Woo in
the years following. Arthur Compton earned the 1927 Nobel Prize in Physics for the
discovery.
The effect is important because it demonstrates that light cannot be explained purely as a
wave phenomenon. Thomson scattering, the classical theory of an electromagnetic wave
scattered by charged particles, cannot explain low intensity shift in wavelength
(Classically, light of sufficient intensity for the electric field to accelerate a charged
particle to a relativistic speed will cause radiation-pressure recoil and an associated
Doppler shift of the scattered light[2], but the effect would become arbitrarily small at
sufficiently low light intensities regardless of wavelength.). Light must behave as if it
consists of particles in order to explain the low-intensity Compton scattering. Compton's
experiment convinced physicists that light can behave as a stream of particle-like objects
(quanta) whose energy is proportional to the frequency.
The interaction between electrons and high energy photons comparable to the rest energy
of the electron (511 keV) results in the electron being given part of the energy (making it
recoil), and a photon containing the remaining energy being emitted in a different
direction from the original, so that the overall momentum of the system is conserved. If
the photon still has enough energy left, the process may be repeated. In this scenario, the
electron is treated as free or loosely bound. Experimental verification of momentum
conservation in individual Compton scattering processes by Bothe and Geiger as well as
by Compton and Simon has been important in disproving the BKS theory.
If the photon is of lower energy, but still has sufficient energy (in general a few eV to a
few keV, corresponding to visible light through soft X-rays), it can eject an electron from
its host atom entirely (a process known as the photoelectric effect), instead of undergoing
Compton scattering. Higher energy photons (1.022 MeV and above) may be able to
bombard the nucleus and cause an electron and a positron to be formed, a process called
pair production.
Contents
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1 Description of the phenomenon
o 1.1 Derivation of the scattering formula
2 Applications
o 2.1 Compton scattering
o 2.2 Inverse Compton scattering
3 See also
4 Notes
5 Further reading
6 External links
[edit] Description of the phenomenon
A photon of wavelength λ comes in from the left, collides with a target at rest, and a new
photon of wavelength λ′ emerges at an angle θ.
See also: Klein-Nishina formula
By the early 20th century, research into the interaction of X-rays with matter was well
underway. It was known that when a beam of X-rays is directed at an atom, the beam
ejects an electron and is scattered through an angle θ. Classical electromagnetism predicts
that the wavelength of scattered rays should be equal to the initial wavelength;[3]
however, multiple experiments found that the wavelength of the scattered rays was
greater than the initial wavelength.[3]
In 1923, Compton published a paper in the Physical Review explaining the phenomenon.
Using Max Planck's notion of quantized radiation and the dynamics of special relativity,
Compton derived the relationship between the shift in wavelength and the scattering
angle:
where
λ is the initial wavelength,
λ′ is the wavelength after scattering,
h is the Planck constant,
me is the mass of the electron,
c is the speed of light, and
θ is the scattering angle.
The quantity h/mec is known as the Compton wavelength of the electron; it is equal to
2.43×10−12 m. The wavelength shift λ′ − λ is at least zero (for θ = 0°) and at most twice
the Compton wavelength of the elctron (for θ = 180°).
Compton found that some X-rays experienced no wavelength shift despite being scattered
through large angles; in each of these cases the photon failed to eject an electron.[3] Thus
the magnitude of the shift is related not to the Compton wavelength of the electron, but to
the Compton wavelength of the entire atom, which can be upwards of 10 000 times
smaller.
[edit] Derivation of the scattering formula
A photon γ with wavelength λ is directed at an electron e in an atom, which is at rest. The
collision causes the electron to recoil, and a new photon γ′ with wavelength λ′ emerges at
angle θ. Let e′ denote the electron after the collision.
From the conservation of energy,
Compton postulated that photons carry momentum;[3] thus from the conservation of
momentum, the momenta of the particles should be related by
assuming the initial momentum of the electron is zero.
The photon energies are related to the wavelengths by
where h is the Planck constant. From the relativistic energy-momentum relation, the
electron energies are
Along with the conservation of energy, these relations imply that
Then
Energies of a photon at 500 keV and an electron after Compton scattering.
From the conservation of momentum,
Then by making use of the scalar product,
Thus
The relation between the frequency and the momentum of a photon is pc = hf, so
Now equating 1 and 2,
Then dividing both sides by 2hff′mec,
Since fλ = f′λ′ = c,
[edit] Applications
[edit] Compton scattering
Compton scattering is of prime importance to radiobiology, as it happens to be the most
probable interaction of high energy X rays with atoms in living beings and is applied in
radiation therapy.[4]
In material physics, Compton scattering can be used to probe the wave function of the
electrons in matter in the momentum representation.
Compton scattering is an important effect in gamma spectroscopy which gives rise to the
Compton edge, as it is possible for the gamma rays to scatter out of the detectors used.
Compton suppression is used to detect stray scatter gamma rays to counteract this effect.
[edit] Inverse Compton scattering
Inverse Compton scattering is important in astrophysics. In X-ray astronomy, the
accretion disk surrounding a black hole is believed to produce a thermal spectrum. The
lower energy photons produced from this spectrum are scattered to higher energies by
relativistic electrons in the surrounding corona. This is believed to cause the power law
component in the X-ray spectra (0.2-10 keV) of accreting black holes.
The effect is also observed when photons from the cosmic microwave background move
through the hot gas surrounding a galaxy cluster. The CMB photons are scattered to
higher energies by the electrons in this gas, resulting in the Sunyaev-Zel'dovich effect.
Observations of the Sunyaev-Zel'dovich effect provide a nearly redshift-independent
means of detecting galaxy clusters.
[edit] See also
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Thomson scattering
Klein-Nishina formula
Photoelectric effect
Pair production
Timeline of cosmic microwave background astronomy
Peter Debye
Walther Bothe
List of astronomical topics
List of physics topics
Washington University in St. Louis (Site of discovery)