Question

UNDER WHAT CONDITION A GLASS LENS PLACED IN A TRANSPARENT LIQUID BECOME INVISIBLE ?

Answer 1

When refractive index of the material of lens is equal to the refractive index of the liquid, there would be no bending of light when it travels from the lens to the liquid and both will start behaving as if they are same entities.Hence, the effect of the lens would dissapear or we can say that the lens becomes invisible.

Answer 2

A lens placed in a transparent liquid becomes invisible because when refractive index of the material of the lens is equal to the refractive index of the liquid in which lens is placed under this condition no bending of light takes place when it travels from liquid to the lens, so both will start behaving like both are same things.
By this, the effect of the lens will vanished so we can say that lens is invisible.