Topics in Particle and Dispersion Science

Relative particle size - or size parameter, is the size of a particle relative to the wavelength of light in the medium surrounding the particle. In the case of a sphere, the relative size is defined as x = 2πa m' / λ₀, where a is the sphere radius, m' is the real part of the refractive index of the medium, and λ₀ is the wavelength of light in vacuum. The relative particle size is also denoted by α.

Although it is implied by the definition of x, the absolute particle size and wavelength are not strictly interchangeable in light scattering theories (see Mie theory: Algorithms for functions of the sphere size and refractive index).