(a) Multiple scattering of a plane wave with a wave vector ki at points 1, 2, …, n-1, n changes its wave vector to kf. The angular distribution of multiply scattered intensity I(kf) results from the interference of waves scattered along all possible paths. It is a random function of kf because the phases of different waves differ by more that 2π and hence can be considered random. This random intensity distribution is called a “speckle pattern.” (b) When the speckle pattern I(kf) is averaged over many realizations of the disordered potential, its random spatial structure washes out, whereas a peak builds up in the backscattering direction kf=-ki. This coherent backscattering (CBS) results from the constructive interference of time-reversed waves that follow the same scattering path in opposite directions.