Multiple scattering occurs when a disturbance encounters more than one scatterer, or revisits scatterers after earlier deflections. Each object is driven by both the original incident wave and fields produced by other objects. Solving the coupled responses therefore differs from adding independent single-scattering intensities.
When wave phases remain correlated, alternative paths must be added as complex amplitudes before squaring, retaining their interference. If suitable randomization makes those cross terms negligible, transport may instead be approximated by a sequence of random deflections; the relevant mean free path compares the typical distance between encounters with sample size. A thick cloudy sample can transmit diffuse light even when its direct beam is weak. Under appropriate coherent conditions, repeated wave scattering also produces weak localization or Anderson localization, which cannot be inferred from independent path probabilities alone.
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