Let I0 be intensity of unpolarised light incident on first polaroid.
I1= Intensity of light transmitted from 1st polaroid
=2I0
If θ be the angle between 1st and 2nd polaroid and ϕ be the angle between 2nd and 3rd polaroid, then
θ+ϕ=90∘...(1)
If I2= Intensity from 2nd polaroid,
I2==I1cos2θ2I0cos2θ
If I3= Intensity from 3rd polaroid
I3======I2cos2ϕI1cos2θcos2ϕ2I0cos2θcos2(90∘−θ)2I0cos2θsin2θ2I0[22sinθcosθ]28I0sin22θ
Now, I3 will be maximum when
sin2θ=1
⇒2θ=90∘
⇒θ=45∘