Rayleigh scattering

In Rayleigh scattering , electrons are excited to a virtual state below the first energy level above ground (Figure 7.5). Electrons quickly return to the ground state, releasing the absorbed photon. The incident and the emitted photon are identical in terms of energy, and hence wavelength/frequency. The measurement procedure can thus be classified as elastic. Since the incident and the emitted photons are identical, special methods have to be used to distinguish them, by polarization, for example.

Figure 7.5: Schematic drawing of energy transfer in a Rayleigh scattering process.

The utility of Rayleigh scattering can be seen from the two following results. First, the intensity of light scattered from small particles scales as

	(7.4)

Here, is a constant, is particle diameter, and is the wavelength of the incident radiation. Equation 7.5 shows that scattering is biased towards shorter wavelengths, blue in favor of red, for example. It can be used to interpret scattered radiation data from the atmosphere where a large number of atomic-scale nuclei are present. Equation 7.5 can be written in an alternative form

	(7.5)

Here, is a constant that depends on wavelength and the geometrical arrangement of the source, object, and the receiver. Intensities and are scattered and incident values respectively. The symbol is the number density of the scattering particles. The number density, analogous to the density of the medium, depends on pressure, temperature, and species concentration. Equation 7.5 can be interpreted as a method of finding temperature when all other quantities are prescribed. Alternatively, it is a method of finding the species concentration, for example in air. See Figure 7.6 and the accompanying article 3 for an example that includes Mie scattered images as well.

Figure 7.6: Rayleigh scattering images in an over-expanded air jet from a converging-diverging nozzle with water added as a scattering medium (adapted from Kim et al., 1998, article 3 ).