When an EM wave passes through an interface of two different isotropic media with different refractive indices, it is partially reflected and transmitted. The amount of reflection and transmission is described by Snell’s Law (also known as Snell-Descartes Law and the law of refraction).

1. Critical Angle: When the light passes from a denser to a rarer medium, for a particular angle of incidence there is no transmitted light – this is referred to as critical angle. By varying the angle of incidence, the transmission is monitored visually.
2. Apparent depth : When an object placed in a denser medium (water in this case),is viewed from rarer medium(air) appears to be lesser in depth than its actual depth. The apperent depth depends on the refractive index of the liquid. $$n= \frac{Actual\:depth}{Apperent\:depth}$$ The apparent depth is measured for different actual depths.