Solution Let I0 be the intensity of polarised light after passing through
the first polariser P1 Then the intensity of light after passing through
second polariser P2 will be
2
0cos
I
I
θ
=
,
where q is the angle between pass axes of P1 and P2 Since P1 and P3
are crossed the angle between the pass axes of P2 and P3 will be
(p/2–q) Hence the intensity of light emerging from P3 will be
I
=I


0
2
2
2
cos
θcos
θ
π –
  = I0 cos2q  sin2q =(I0/4) sin22q
Therefore, the transmitted intensity will be maximum when q = p/4