Combining colloidal-probe experiments and computer simulations, we analyze the solvation forces F of charged silica colloids confined in films of various thicknesses h. We show that the oscillations characterizing F(h), for sufficiently large h, are determined by the dominant wavelength of the bulk radial distribution function. As a consequence, both quantities display the same power-law density dependence. This is the first direct evidence, in a system treatable both by experiment and by simulation, that the structural wavelength in bulk and confinement coincide, in agreement with predictions from density functional theory. Moreover, theoretical and experimental data are in excellent quantitative agreement.