Question

A biconcave symmetric lens is placed inside a medium whose refractive index is $n_{medium}$ with respect to the air. The lens acts as a converging lens (convex lens) in this medium. So what can you say about the refractive index of the lens ($n_{lens}$) with respect to the air?

• A. $n_{lens}$ > $n_{medium}$
• B. $n_{lens}$ < $n_{medium}$
• C. $n_{lens}$ = $n_{medium}$
• D. cannot be predicted

Answer 1

The correct option is B $n_{lens}$ < $n_{medium}$

If the lens is optically rarer as compared to the medium (i.e. $n_{lens}$ < $n_{medium}$) then, when light travels from the medium to the lens, it will move away from the normal. Therefore, the lens will act as a converging lens. If $n_{lens}$ > $n_{medium}$ or $n_{lens}$ = $n_{medium}$, then light will never converge from the concave lens.