Abstract
A method of testing the validity of the Rayleigh assumption is presented. The test can be applied for any one-dimensional or two-dimensional surface of analytic shape. It can be used for Dirichlet and Neumann boundary conditions and also when the surface is penetrable and a refracted field exists. The test is based on the asymptotic evaluation of the diffracted order amplitudes and in this way is similar to Millar's potential-theory analysis.