Re: How is the refractive index of a glass prism measured by the

Ellipsometry is generally used to measure the refractive index of very
thin films (eg. microns in thickness).  The basic principle is as follows:
polarised light is shone onto a reflective surface, and the effect of the
reflection is measured.  When the surface is coated with a thin film, the
effect of the reflection on the polarization properties is altered,
providing a sensitive measure of the refractive index. The input and 
output polarization vectors can be used to define a diagnostic ellipse
(representing the change in amplitude and phase), giving the name to the
technique.

Prism coupling is a complementary technique generally applied to 
thicker layers.  The principle of the method is similar to ellipsometry,
but here the incident beam is shone first through a prism which is
in direct contact with the film.  At the correct angles, determined by
the film properties, it acts as a waveguide whilst at other angles the
light is internally reflected within the prism.  If the properties of the
prisms are known precisely, then very accurate measurements of refractive
index can be made.