A relation between the diffusion coefficient D and the distance λ‎ that a particle can jump when diffusing in a time τ‎. The Einstein–Smoluchowski equation, which is D = λ‎²/2τ‎, gives a connection between the microscopic details of particle diffusion and the macroscopic quantities associated with the diffusion, such as the viscosity. The equation is derived by assuming that the particles undergo a random walk. The quantities in the equation can be related to quantities in the kinetic theory of gases, with λ/τ‎ taken to be the mean speed of the particles and λ‎ their mean free path. The Einstein–Smoluchowski equation was derived by Albert Einstein and the Polish physicist Marian Ritter von Smolan-Smoluchowski (1872–1917).