Scattering

Scattering takes place when streams of photons (or waves of light) are deflected in different directions.
In this resource, the term is used to refer to the different forms of deviation produced by diffusion, dispersion, interference patterns, reflection and refraction as well as by the composition and surface properties of different media.

Random scattering
  • In optics, diffusion results from any material that scatters light during transmission or reflection producing softened effects without sharp detail.
  • Objects produce diffuse reflections when light bounces off a rough or uneven surface and scatters in all directions.
  • Transparent and translucent materials transmit diffuse light unless their surfaces are perfectly flat and their interiors are free of foreign material.
  • All objects obey the law of reflection on a microscopic level, but if the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion.
  • A reflection that is free of the effects of diffusion is called a specular reflection.
  • In the case of raindrops, random scattering can result from:
    • Atmospheric conditions affecting incident sunlight.
    • Turbulence distorting the shape of raindrops.
    • Light being reflected off the surface of multiple raindrops, one after another, before reaching an observer.
Regular scattering

Raindrops produce a regular form of scattering. Let’s explore them by way of example of regular scattering:

  • When light of a particular wavelength strikes the surface and enters a raindrop its subsequent path depends on its original point of impact, the refractive indices of air and water and the surface properties of the droplet.
  • For incident rays of a single wavelength striking the surface of a single droplet at different points, it is the different angles at which they enter the droplet that is the chief determinant of the way they scatter as they exit the droplet.
  • For incident rays of white light striking the surface of a single droplet at different points, it is the combined effects of the different angles at which they enter the droplet along with the effects of chromatic dispersion (causing the separation of white light into spectral colours) that determine the form of scattering.
  • Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
  • Regular scattering is not random and obeys the law of reflection and refraction (Snell’s law).

Summary

Scattering in physics

Rayleigh scattering refers to the scattering of visible light or other electromagnetic radiation by particles smaller than the wavelength of the radiation. Rayleigh scattering is wavelength-dependent.

Mie scattering refers to the scattering of visible light or other electromagnetic radiation by particles larger than the wavelength of the radiation. Mie scattering is wavelength-dependent. Mie scattering is responsible for the white appearance of the clouds.

Non-selective scattering is similar to Mie scattering and takes place when the particles are much larger than the incident radiation. This type of scattering is not wavelength dependent and is the primary cause of atmospheric haze.