## Ellipsometry

Here is my situation:
According to ellipsometry theory the complex reflectance ratio can be expressed as

where tan($$\Psi$$) is the amplitude ratio upon reflection and $$\Delta$$ is a phase shift.
On the other hand, as far as I now, phase shift upon reflection is $$\pi$$ or 0, depending on materials.
the question: is the $$\Psi$$ only variable that is being changed in wide range while $$\Delta$$ is either $$\pi$$ or 0 in all cases.

Sorry for not following subforum format, I just could not fit my question in it.
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 Phase shifts of pi and 0 occur at normal incidence for s or p polarized light. Ellipsometry uses oblique angles of reflections and compares the phase shifts after reflection of the two states of polarized light. See the below link for a brief tutorial of ellipsometry. http://www.jawoollam.com/tutorial_1.html
 So it turnes out that s and p polarized lights are phase shifted differently upon reflection. but why?