English: 1909 thought experiment used by Einstein to demonstrate wave-particle duality. Einstein imagined a mirror in a cavity containing particles of an ideal gas and black body radiation, with the entire system in thermal equilibrium. The mirror is constrained in its motions to a direction perpendicular to its surface. The mirror jiggles from Brownian motion due to the gas molecules. Since the mirror is in a radiation field, the jiggling mirror transfers some of its kinetic energy to the radiation field. This implies that there must be fluctuations in the black body radiation field, and hence fluctuations in the black body radiation pressure. Reversing the argument shows that there must be a route for the return of energy from the fluctuating black body radiation field to the gas. Given the known shape of the black body radiation field given by Planck's Law, Einstein could calculate the various contributions to the fluctuations of the mirror's motions. The expression of the radiation contribution was the sum of two terms: one due to the wave-like properties of radiation, the other due to its particulate aspects. Neither individual term was sufficient in itself. From this, Einstein concluded that radiation had simultaneous wave and particle aspects.