JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Wave Optics

As shown in figures, three identical polaroids

P_{1}

P_{2}

and

P_{3}

are placed one after another. The pass axis of

P_{2}

and

P_{3}

are inclined at angle of

6 0^{\circ}

and

9 0^{\circ}

with respect to axis of

P_{1}

. The source S has an intensity of

256 \frac{W}{m ^{2}}

. The intensity of light at point O is ______

\frac{W}{m ^{2}}

Answer: 24

Solution

Unpolarised light loses half its intensity through the first polaroid, so after

P_{1}

the intensity is

\frac{256}{2} = 128

W m

^{- 2}

P_{2}

is at

6 0^{\circ}

P_{1}

, so by Malus's law

128 cos^{2} 6 0^{\circ} = 32

W m

^{- 2}

P_{3}

is at

9 0^{\circ}

P_{1}

, hence

3 0^{\circ}

from

P_{2}

, giving

32 cos^{2} 3 0^{\circ}

. The intensity reaching

O

24

W m

^{- 2}

JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Integer answer

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