  The contact angle of a liquid drop on a rigid surface is determined by the
classical theory of Young-Laplace. For chemically homogeneous surfaces, this
angle is a constant.
  We study the minimal-energy configurations of liquid drops on rough surfaces.
Here the actual angle is still constant for homogeneous surfaces, but the
apparent angle can fluctuate widely. A limit theorem is introduced for minimal
energy configuration, where the rigid surface converges to a smooth one, but
the roughness parameter is kept constant. It turns out that the limit of
minimal energy configurations correspond to liquid drop on a smooth surface
with an appropriately defined effective chemical interaction energy. It turns
out that the effective chemical interaction depends linearly on the roughness
in a certain range of parameters, corresponding to full wetting. Outside this
range the most stable configuration corresponds to a partial wetting and the
effective interaction energy depends on the geometry in an essential way.
  This result partially justifies and extends Wenzel and Cassie's laws and can
be used to deduce the actual inclination angle in the most stable state, where
the apparent one is known by measurement. This, in turn, may be applied to
deduce the roughness parameter if the interfacial energy is known, or visa
versa.
