18 can be easily understood by assuming
that light passing through the polaroid P2 gets polarised along  the pass-
axis of  P2 If the pass-axis of  P2 makes an angle q with the pass-axis of
P1, then  when the polarised beam passes through the polaroid P2, the
component  E cos q  (along the pass-axis of P2) will pass through P2 Thus, as we rotate the polaroid P1 (or P2), the intensity will vary as:
I = I0 cos2q
(10 18)
where I0 is the intensity of the polarized light after passing through
P1