The critical angle between an equilateral prism and air is $45°$. If the incident ray is perpendicular to the refracting surface, then

The minimum refractive index of a material of a prism of apex angle $90°$ for which light cannot be transmitted for any value of $i$ is,

A horizontal light ray passes through a prism $(\mu =1.5)$ of angle $4°$. Further, it is incident on a plane mirror $M,$ that has been placed vertically. By what angle the mirror is rotated so that the ray after reflection becomes horizontal?

An equilateral prism is kept on a horizontal surface. A typical ray of light $PQRS$ is shown in the figure. For minimum deviation