Which of the following phenomena can be demonstrated by light,but not with sound waves in an air column?

$A$ calcite crystal is placed over a dot on a piece of paper and rotated. On seeing through the calcite,one will see:

Polarized glass is used for sunglasses because...

$A$ beam of plane polarised light of large cross-sectional area and uniform intensity of $3.3 \, W m^{-2}$ falls normally on a polariser (cross-sectional area $3 \times 10^{-4} \, m^2$) which rotates about its axis with an angular speed of $31.4 \, rad/s$. The energy of light passing through the polariser per revolution is close to ........ $\times 10^{-4} \, J$.

The tourmaline crystal:

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 an unpolarised beam of light of intensity $128 \ Wm^{-2}$ is incident on $P_{1}$,what is the intensity of light coming out of $P_{3}$?