Photon diffusion

Photon diffusion is an approximate description of how light spreads through a material in which scattering is much more common than absorption. In such media, photons undergo many successive scattering events, which repeatedly change their direction of travel. Over many interactions, the path of an individual photon can be described statistically as a random walk.

When large numbers of photons are considered together, their overall transport behaves in a way similar to diffusion. In this regime, the distribution of light energy spreads through the material in a manner that can be described using a diffusion equation. This approximation is derived from the more general radiative transfer equation and is valid when the material is optically thick and scattering dominates over absorption.

This description is commonly used in modeling light propagation in media such as biological tissue, clouds, and other materials where repeated scatterings strongly influences how light moves through the system.