 A thin metal film (for example gold) is positioned close enough to the prism wall so that an evanescent wave can interact with the plasma waves on the surface and hence excite the plasmons.In the Kretschmann configuration (also known as Kretschmann-Raether configuration), the metal film is evaporated onto the glass block.The light again illuminates the glass block, and an evanescent wave penetrates through the metal film.