Absorption technique

The special case of absorption, interpreted as a scattering technique is discussed in the present section. Several gases and liquids are practically transparent to visible radiation. The transmittivity is close to unity, the associated absorptivity being close to zero. In the presence of suspended particles, light is scattered and transmittivity is no longer unity. Scattering is generally uniform in all directions, unless the particle is transparent or unsymmetric in shape. In the former, considerable anisotropy is generated by refraction, internal and external, in radiation leaving the particle surface. Scattering redirects energy from a given direction to others. Consequently, energy transmitted in a particular direction is reduced. One can then think of scattering as being equivalent to absorption of energy.

Even in the absence of particles, molecules of a medium – solid, liquid or gas, can scatter radiation at specific wavelengths. Thus, when radiation passes through the physical medium, it is possible that the material shows non-zero absorptivity for certain wavelengths, while being purely transparent to others.

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Figure 7.2 Absorption of radiation in a gaseous medium. The incoming energy is denoted as while changes are calculated over a gas layer thickness of .