Water in a pond appears to be only three-quarter of its actual depth. (a) What property of light is responsible for this observation? Illustrate your answer with the help of a ray diagram. (b) How the refractive index of water calculated from its real and apparent depths?

(a) 'Refraction of light' is responsible for this property.

(b) When light coming from denser medium enters the rarer medium, it is bent away from the normal.

Let any object B is at the bottom of a pond. Consider a light ray BC from the object that moves from water to air. After refraction from the water surface, the ray moves away from the normal N along the path CD. The produce of CD appears from the point B' and a virtual image of the object at B appears at B'.

(c) The pond appears to be three-quarters of its actual depth due to the property of refraction of light.

​​​​​​Refractive index $\mu =\frac{\text{Real depth}}{\text{Apparent depth}}$

Since $\mu =\frac{4}{3}$ for water

Therefore, Apparent depth = $\frac{4}{3}\times$ Real depth