Everything about Thomson Scattering totally explained
In
physics,
Thomson scattering is the scattering of
electromagnetic radiation by acharged particle. The
electric and
magnetic components of the
incident wave accelerate the particle. As it
accelerates, it in turn emits
radiation and thus, the wave is scattered. Thomson scattering is an important
phenomenon in
plasma physics and was first explained by the physicist
J.J. Thomson.
As long as the motion of the particle is non-
relativistic (for example its speed
is much less than the speed of light), the main cause of the acceleration of the particle will be due
to the electric field component of the incident wave. The particle will move in
the direction of the oscillating electric field, resulting in
electromagnetic dipole radiation. The moving particle radiates most strongly in a direction
perpendicular to its motion and that radiation will be
polarized along the
direction of its motion. Therefore, depending on where an observer is located,
the light scattered from a small volume element may appear to be more or less
polarized.
The electric fields of the incoming and observed beam can be divided up into
those components lying in the plane of observation (formed by the incoming and
observed beams) and those components perpendicular to that plane. Those
components lying in the plane are referred to as "radial" and those
perpendicular to the plane are "tangential", since this is how they appear to
the observer.
The diagram on the right is in the plane of observation. It shows the radial component of the incident electric field causing a component of motion of the charged particles at the scattering point which also lies in the plane of observation. It can be seen that the amplitude of the wave observed will be proportional to the cosine of χ, the angle between the incident and observed beam. The intensity, which is the square of the
amplitude, will then be diminished by a factor of cos
2(χ). It can be seen
that the tangential components (perpendicular to the plane of the diagram) won't be affected in this way.
The scattering is best described by an
emission coefficient which is defined
as ε where ε dt dV dΩ dλ is the energy
scattered by a volume element
in time dt into solid angle dΩ
between wavelengths λ and λ+dλ. From the point of view of
an observer, there are two emission coefficients, ε
r corresponding to
radially polarized light and ε
t corresponding to tangentially polarized
light. For unpolarized incident light, these are given by:
»
Examples of Thomson scattering
The
cosmic microwave background is thought to be linearly polarized as a result of Thomson scattering. Probes such as
WMAP and the future
Planck mission attempt to measure this polarization.
The solar
K-corona is the result of the Thomson scattering of solar radiation from solar coronal electrons. NASA's
STEREO mission will generate three-dimensional images of the electron density around the sun by measuring this K-corona from two separate satellites.
In
tokamaks and other experimental
fusion devices, the electron temperatures and densities in the
plasma can be
measured with high accuracy by detecting the effect of Thomson scattering of a high-intensity
laser beam.
Further Information
Get more info on 'Thomson Scattering'.
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