Scattering of Electromagnetic Radiation
Scattering of electromagnetic radiation is caused by the interaction of radiation
with matter resulting in the reradiation of part of the energy to other directions
not along the path of the incidint radiation. Scattering effectively removes energy
from the incident beam. Unlike absorption, this energy is not lost, but is redistributed
to other directions.
Both the gaseous and aerosol components of the atmosphere cause scattering in the atmosphere.
Scattering by gaseous molecules
The law of scattering by air molecules was discovered by Rayleigh in 1871, and
hence this scattering is named Rayleigh Scattering. Rayleigh scattering
occurs when the size of the particle responsible for the scattering event is much
smaller than the wavelength of the radiation. The scattered light intensity is
inversely proportional to the fourth power of the wavelength. Hence, blue light
is scattered more than red light. This phenomenon explains why the sky is blue
and why the setting sun is red.
The scattered light intensity in Rayleigh scattering for unpolarized light
is proportional to (1 + cos2 s) where s is the
scattering angle, i.e. the angle between the directions of the incident and
Scattering by Aerosols
Scattering by aerosol particles depends on the shapes, sizes and the materials
of the particles. If the size of the particle is similar to or larger than the
radiation wavelength, the scattering is named Mie Scattering. The scattering
intensity and its angular distribution may be calculated numerically for a spherical
particle. However, for irregular particles, the calculation can become very complicated.
In general, the scattered radiation in Mie scattering is mainly confined within
a small angle about the forward direction. The radiation is said to be very
strongly forward scattered.
The Earth's Atmosphere
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