Question

Three polaroid sheets $P_1,P_2$ and $P_3$ are kept parallel to each other such that the angle between pass axes of $P_1$ and $P_2$ is $45^{\circ }$ and that between $P_2$ and $P_3$ is $45^{\circ }$. If unpolarised beam of light of intensity $128W{m}^{-2}$ is incident on $P_1$. What is the intensity of light coming out of $P_3$?

• A. $128W{m}^{-2}$
• B. 0
• C. $16W{m}^{-2}$
• D. $64W{m}^{-2}$

Answer 1

Answer: (C)

We have unpolarised beam's intensity
$I_0=128w/m^2$
Using Malu's law we have
$I=I_0{\cos }^2\theta$
When beam passed from the first polaroid,
$I=I_0/2$
Again, as the angle between $p_1$ and $p_2$ is $45^{\circ }$,
beam intensity when it will pass $p_2$ would be
$l_1=I_0/2{\cos }^{2}45^0=I_0/4$
And, also the angle between $p_2$ and $p_3$ is $45^{\circ }$,
beam intensity when it will come out of $p_3$ will be
$l_2=I_0/4{\cos }^{2}45^0=I_0/8$
$=128/8=16w/m^2$