This free-surface–turbulence interaction may also be characterized by a wavenumber spectrum. If "δ" is the instantaneous displacement of the surface from its average position, the mean squared displacement may be represented with a displacement spectrum "G"("k") as:  A three dimensional form of the pressure spectrum may be combined with the Young–Laplace equation to show that:  Experimental observation of this "k"−19/3 law has been obtained by optical measurements of the surface of turbulent free liquid jets.