An air bubble in a glass slab of thickness $15 \, cm$ appears to be at $6 \, cm$ from one side and at $4 \, cm$ from the opposite side. The refractive index of the glass is:

$A$ bird is flying $3\, m$ above the surface of water. If the bird is diving vertically down with speed $= 6\, m/s$,its apparent velocity as seen by a stationary fish underwater is $m/s$.

$A$ composite slab consisting of different media is placed in front of a concave mirror of radius of curvature $150 \, cm$. The whole arrangement is placed in water (refractive index $\mu = 4/3$). An object $O$ is placed at a distance $20 \, cm$ from the slab. The refractive indices of the different media are given in the diagram. Find the final image formed by the system.

Obtain the relation between the real depth and apparent depth of the bottom of a tank filled with water when observed from air.

$A$ plane glass slab is kept over various coloured letters. The letter which appears least raised is

To determine the refractive index of a glass slab using a travelling microscope, the minimum number of readings required is: