The formulae enabling us to calculate the values of the ellipsometric parameters of randomly rough surfaces and very thin films with randomly rough boundaries with respect to the influence of the shadowing between the irregularities of roughness are presented. These formulae are derived within the scalar diffraction theory of light. The first formula allows the calculation of the ellipsometric parameters using a numerical method, while the second formula is the approximative one but it expresses the ellipsometric parameters in a closed form. The numerical analysis of both formulae is performed for several examples of randomly rough surfaces and thin films. Moreover, the comparison of the results achieved using both formulae respecting the shadowing and an earlier formula not including this effect is performed. The experimental data of two samples of the randomly rough silicon surfaces covered with very thin surface layers are interpreted using all the formulae mentioned. Using this experimental study, the correctness of both formulae taking into account the shadowing, is confirmed.