The light waves incident on the surface of the water at an incident angle greater than critical angle will get reflected in side water. This light waves forms cone in the volume of the tank. Thus we get relation of critical angle as tanθc = R/h.
Rays coming out of the source and incident at an angle greater than θc will be reflected back into the liquid; therefore, the corresponding region on the surface will appear dark. As it is obvious from the figure, the radius of the bright spot is given by