The problem of light depolarization in a turbulent atmosphere is revisited by means of coupled-mode theory [D. Marcuse, Theory of Dielectric Optical Waveguides (Academic, New York, 1974)]. This allows, in particular, evaluation of the depolarization ratio for a plane wave and comparison of its expression with the one obtained in the frame of two distinct approaches predicting different behaviors [A. A. M. Saleh, IEEE J. Quantum Electron. QE-3, 540 (1967); J. W. Strohbehn and S. F. Clifford. IEEE Trans Antennas Propag. AP-15,416 (1967)]. It is shown that both approaches give the same result when calculated to the same order in both smallness parameters relevant to the problem, thus resolving a controversy of more than 20 years.