Agreement between the theoretical and measured resolution dependence of x-ray specular reflection from the ${\mathrm{H}}_2$O-vapor interface shows that the macroscopic capillary model for surface roughness can be extended to length scales as small as 400 Å. Agreement between measured thermal diffuse scattering data and the theoretical form, with no significant adjustable parameters, independently leads to similar conclusions.

• Received 8 December 1989

DOI:https://doi.org/10.1103/PhysRevA.41.5687