Question

A convex lens is placed in a medium in which it behaves like a glass plate. The refractive index of the medium, will be

• A. equal to refractive index of air
• B. more than the refractive index of glass
• C. equal to the refractive index of glass
• D. less than the refractive index of glass

Answer 1

Answer: (C)

equal to the refractive index of glass

Answer 2

We know from the lens maker's formula the focal length, $\frac{1}{f}=\left(\frac{\mu_{2}-\mu_{1}}{\mu_{1}}\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ Now for the lens to behave like a glass plate its focal length will be infinity $(f=\infty)$ $\therefore \mu_{2}-\mu_{1}=0$ $\Rightarrow \mu_{2}=\mu_{1}$ That is, the refractive index of the medium should be equal to the refractive index of glass.