where θ θ is the angle of incidence. An extra factor of π π is sometimes erroneously included in the exponent of the above equation. At normal incidence, θ θ = 0 , perfectly specular scattering (i.e. p ( λ λ ) = 1 ) would require an arbitrarily large wavelength, or conversely an arbitrarily small roughness. Purely specular scattering does not introduce a boundary-associated increase in the thermal resistance. In the diffusive limit, however, at p = 0 the relaxation rate becomes