Rayleigh scattering (contd...)

Rayleigh scattering is at the interface of wave optics and quantum optics and the properties of the scattered signal can also be determined by the classical electromagnetic field (Maxwell) equations.

When a light beam of strong enough intensity falls on a small portion of a fluid region, the scattered light will contain light intensity fluctuations that are thermal in origin. This effect may not be as pronounced in solids where a lattice structure can create phase differences that result in destructive interference. Phase becomes a near-random variable in liquids and gases, owing to Brownian motion, and a persistent noisy signal is obtained. The statistical properties of the scattered signal, specifically the decay of the autocorrelation function can be related to those in the dielectric constant, and hence the material density. This association is possible because Rayleigh scattering can be explained from a quantum mechanical point-of-view as well as from electromagnetic field theory. Since these fluctuations are thermal in origin, the decay constant of the autocorrelation function will scale with thermal diffusivity of the material. The entire approach is called dynamic light scattering and is useful for thermal diffusivity measurements of fluids under a wide range of conditions. Specific advantages of this route include the small sample size, small experimental time duration, and the absence of any calibration. The method requires detection of small light intensities, adequately resolved in time, and followed by statistical analysis of the light fluctuations (recorded with a high speed detector as a time series of voltages).