Why do s and p polarized lights have different phase shifts in ellipsometry?

[geo_alchemist]

Here is my situation:
According to ellipsometry theory the complex reflectance ratio can be expressed as
[PLAIN]http://img255.imageshack.us/img255/2657/68578079.png
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.

 

[chrisk]

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

 

[geo_alchemist]

So it turnes out that s and p polarized lights are phase shifted differently upon reflection.
but why?