A concave mirror of focal length `20cm` is placed inside water with its shining surface upwards and principal axis vertical as shown in Fig. Rays are incident parallel to the principal, axis of concave mirror. Find the position of final image.

Explanation

The incident rays will pass undeviated throught the water surface and strike the mirror parallel to its principal axis. Therefore, for the mirror, object is at `oo`. Its image A(in figure Fig.) will be formed at focus which is 20 cm from the mirror. Now, for the interface between water and air. For observer at air, the image formed by mirror will act as an object for observer with real depth `d=30-20=10cm`.
Apparent depth observed by observer,
`d^(')=(d)/((n_(w)/(n_(a))))=(10)/(((4//3)/(1)))=7.5cm`