Scattering of Electromagnetic Radiation by Atmosphere

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 scattered rays.

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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Please send comments/enquiries/suggestions about this tutorial to Dr. S. C. Liew at Copyright CRISP, 2001