It depends. Given a definition of the ellipsometric angles ψ and Δ as:
tan(ψ) exp(iΔ) = Rp/Rs,
Where Rp is the reflection coefficient for p-polarized light and Rs the coefficient for s-polarized light. ψ is then related to the ratio of reflectivities and Δ the relative phase shift experienced by s- and p-polarized light.
The reflectivities are given by the (complex) refractive index and angle of incidence, so ψ and Δ depend on both those quantities. Thus, 'nulling ellipsometry' is often used to measure how those angles change when something adsorbs onto the reflective surface by setting the instrument (compensator, polarizer, and analyzer angles), to zero when measuring the bare surface.
Azzam and Bashara's book has infinite detail about ellipsometry.