Scaling laws and universality of critical phenomena in the presence of an external field

Critical singularities disappear in systems placed in an inhomogeneous field that couples with the order parameter. On the basis of simple renormalization arguments we show that the change from singular behavior in the absence of a field to nonsingular behavior in the presence of a field is governed by universal scaling laws. As a example we discuss the scaling properties of the density profile in a fluid close to the vapor-liquid critical point in the presence of a gravitational field. We make also the connection between these scaling properties and the theory of Jasnow and Rudnick for the effect of capillary waves on the vapor-liquid interface. It is shown how these effects depend on the bulk properties of the fluid near the critical point and, hence, how the reflectivity of the interface can be evaluated for different fluids. We discuss also the scaling behavior of the surface tension and the various options for its definition when an external field is present.