JEE Main 26-Aug-2021 Shift 2 Solved Paper

Given, intensity of light falling on screen is I.
Intensity of light when P1 and P2 placed between screen and source is I/2.
As per Malus’s law, the intensity of transmitted light when passing through a pair of polariser and analyser having angle ϕ between their axis is given as
I=I0cos2ϕ Initially let us consider the angle between analyser and polariser is 0°.
Then, the intensity of transmitted light is I / 2. Thus, intensity of light become I / 2 after transmitting through 1st polariser as shown in the figure.
If we rotate the polarise P2 by angle ϕ, then the resulting intensity of light on screen will be 3/∕8.
Now, for second condition, apply Malus’s law,

3l

8

=

l

2

cos2ϕ
cos2ϕ=

3

4

⇒ cos‌ϕ=

√3

2

ϕ = 30°. Thus, P2 should be rotated by 30° to get intensity of light 31/∕8 on screen.