Let `I_(0)` be the intensity of polarised after passing through the first polariser `P_(1)`. Then the intensity of light after passing through second polariser `P_(2)` will be ltbgt `I=I_(0)cos^(2) theta`, where `theta` is the angle between pass axces of `P_(1)` and `P_(2)`. Since `P_(1)` and `P_(2)` are crossed the angle between the pass axes of `P_(2)` and `P_(3)` will be `(pi//2-theta)` . Hence the intensity of light emerging from `P_(3)` will be `I= I_(0)cos^(2) theta cos^(2) ((pi)/(2)-theta)`
`=I_(0)cos^(2) theta= (I_(0)//4)sin^(2) 2theta`
Therefore, the transmitted intensity will be maximum when `theta = pi//4`.
