What is total internal reflection?

Choose the correct option:

Consider the optically denser lenses P, Q, R, and S drawn below. According to Cartesian sign convention which of these have positive focal length?

Two plane mirrors are inclined at angle ${40}^{\circ }$ between them. Number of images seen of a tiny object kept between them is_____.

A concave mirror of curvature $40 \mathrm{cm}$, used for shaving purpose produces an image of double size as that of the object. Object distance must be :

Choose the correct option.

Which of the following aberrations will NOT occur for spherical mirror ?