Which of the following is a biaxial crystal?

An unpolarized light of certain intensity passes through a combination of two polarizers whose transmission axes are at $30^\circ$ and $90^\circ$,respectively,with respect to the horizontal axis. $A$ third polarizer with its transmission axis at $60^\circ$ with the horizontal axis is placed between the two existing polarizers. The ratio of the output intensities with and without the third polarizer is . . . . . . .

How can it be determined whether a given light is unpolarized,plane-polarized,or partially plane-polarized?

Two polaroids $A$ and $B$ are placed in such a way that the pass-axis of polaroids are perpendicular to each other. Now,another polaroid $C$ is placed between $A$ and $B$ bisecting the angle between them. If the intensity of unpolarised light is $I_0$,then the intensity of transmitted light after passing through polaroid $B$ will be:

Unpolarized light of intensity $I$ passes through an ideal polarizer $A$. Another identical polarizer $B$ is placed behind $A$. The intensity of light beyond $B$ is found to be $\frac{I}{2}$. Now,another identical polarizer $C$ is placed between $A$ and $B$. The intensity beyond $B$ is now found to be $\frac{I}{8}$. The angle between polarizer $A$ and $C$ is: ......$^{\circ}$

An optically active compound: