help for optics question

[cuti_pie75]

Here's the question:

a beam of natural light is incident on an air-glass interface (n-ti = 1.5) at 40°. Compute the degree of polarization of the reflected light.

In here, i'm not too sure of the degree of polarization that they're talking about nor which formula to use

Although, i know that the formula for V=(Ip)/(Ip+In) = (Imax-Imin)/(Imax+Imin) but i dunno how i can use this formula, i mean, we don't have the intensity in the pb, the only thing we can find is R// that can be related to I since Ir// = R//Ii/2

(the // means parallel btw) and so far, i figure out all the fresnel coefficient: r//, t//, r (perpendicular) and t(perpendicular). Then dunno what to do next.

Really need help with this...and i'm really lost with what's going on.
Thanx in advance

[Claude Bile]

The degree of polarisation refers to the ratio of reflected p and s polarised irradiances. For example a degree of polarisation of 100 means that the reflected p (or s) polarised irradiance is 100 times greater than the reflected s (or p) irradiance.

Claude.