The angle of polarisation for a medium with respect to air is $60^{\circ}$. The critical angle of this medium with respect to air is

An object is placed at a distance of 18 cm in front of a mirror. If the image is formed at a distance of 4 cm on the other side, then focal length, nature of the mirror and nature of image are respectively

An object lying 100 cm inside water is viewed normally from air. If the refractive index of water is $\frac{4}{3}$, then the apparent depth of the object is

The minimum deviation produced by a hollow prism filled with a certain liquid is found to be $30^{\circ}$. The light ray is also, found to be refracted at an angle of $30^{\circ}$. Then, the refractive index of the liquid is