CBSE Class 12 · 2014 · Delhi · Wave OpticsLong Answer

(a) Show, giving a suitable diagram, how unpolarized light can be polarized by reflection.
(b) Two polaroids

P_{1}

and

P_{2}

are placed with their pass axes perpendicular to each other. Unpolarised light of intensity

I_{0}

is incident on

P_{1}

. A third polaroid

P_{3}

is kept in between

P_{1}

and

P_{2}

such that its pass axis makes an angle of

6 0^{\circ}

with that of

P_{1}

. Determine the intensity of light transmitted through

P_{1}

P_{3}

and

P_{2}

Answer: $I_{P_{1}} = \frac{I _{0}}{2}$ , $I_{P_{3}} = \frac{I _{0}}{8}$ , $I_{P_{2}} = \frac{3 I _{0}}{32}$ .

Solution

(a) When unpolarized light is incident on a transparent medium at the Brewster angle

i_{p} = tan^{- 1} μ

, the reflected ray is completely plane polarized (vibrations parallel to the surface) while the refracted ray is partially polarized; at this angle the reflected and refracted rays are perpendicular.
(b)

P_{1}

receives unpolarized

I_{0}

I_{P_{1}} = \frac{I _{0}}{2}

P_{3}

makes

6 0^{\circ}

with

P_{1}

; by Malus' law

I_{P_{3}} = \frac{I _{0}}{2} cos^{2} 6 0^{\circ} = \frac{I _{0}}{2} \cdot \frac{1}{4} = \frac{I _{0}}{8}

P_{2} ⊥ P_{1}

, so

P_{2}

makes

(9 0^{\circ} - 6 0^{\circ}) = 3 0^{\circ}

with

P_{3}

I_{P_{2}} = I_{P_{3}} cos^{2} 3 0^{\circ} = \frac{I _{0}}{8} \cdot \frac{3}{4} = \frac{3 I _{0}}{32}

I_{P_{1}} = \frac{I _{0}}{2}, I_{P_{3}} = \frac{I _{0}}{8}, I_{P_{2}} = \frac{3 I _{0}}{32}

CBSE Class 12 Board · 2014 · Delhi · Subjective

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CBSE Class 12 · 2014 · Delhi · Wave OpticsLong Answer

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