Question

In a series of polaroids placed in such a way that the optical axis of each polaroid makes the same angle with the next one. When unpolarized light falls on first and if no light is coming out from the last polaroid, then by using how many number of polaroids in series this can be achieved?

• A. $3$
• B. $4$
• C. $5$
• D. $6$

Answer 1

Answer: (A), (B), (C), (D)

$6$

When each Polaroid is placed with its optic axis at ${90}^{\circ }$ with the previous and next Polaroid, then no light will come out.

For $3$ Polaroids in series:


For $4$ Polaroids in series:


Similarly, for $5$ and $6$ Polaroids in series, the same can be easily proved.

Therefore, all the options are correct.

Trick: For $n$ Polaroids, $I_n=I_0{\cos }^{2n}\theta$ Here, $I_n=0$ $\therefore {\cos }^{2n}\theta =0$ $\Rightarrow$ It is valid for any integer value of $n$.