Consider scattering form a surface with r.m.s. height variation of /2, where is the wavelength. Show that the stationary phase model can describe this

Consider scattering form a surface with r.m.s. height variation of λ/2, where λ is the wavelength. Show that the stationary phase model can describe this scattering provided that the incident angle is less than about 60^o and m < 0.60, whereas the scalar model requires m < 0.25, where m is the r.m.s surface slope variation.