For particles with diameters that are not significantly larger than the wavelength of the light used, the Mie theory is applied for the analysis of the measurements. This theory was developed at the start of the 20th century by Gustav Mie and is the complete solution of the Maxwell equations for the scattering of electromagnetic waves by spherical particles.
It can be used to analyse the characteristic intensity distributions for even very small particles, which, in contrast to Fraunhofer theory, are not restricted to scattering angles of less than 90° (forward scattering). In fact, scattering angles of greater than 90° also occur (backward scattering). In order to be able to use the intensity distribution for the calculation of the particle size, determined in this manner, the refraction index and absorption index of the sample must be known with the Mie theory in contrast to the Fraunhofer theory.
The lowest limit of the size range obtainable with the Mie theory, is approximately at 10 nm. For even smaller particles, the scattering intensity is not dependent on the direction, which means from the angle dependency of the scattered light the particle size cannot be any longer calculated. It is considered the so called Rayleigh scattering (which by the way is responsible for the blue colour of the sky).
In this size range the dynamic light scattering is utilized.