You shine a red laser (500 nm wavelength) at a tabletop. (a) What is the surface roughness requirement for a specular reflection at 0 incidence angle? (b) How does this requirement change when the incidence angle is increased to 65?

Note the Rayleigh Criterion: a surface can be defined as smooth enough for scattering to be specular if the phase difference is less than some arbitrarily defined value of the order of 1 radian. The conventional value is /2, and this is called the Rayleigh criterion. Thus, for a surface to be smooth according to this criterion, surface roughness variability (h) < /8cos()