Question

A hollow double concave lens is made of very thin transparent material. It can be filled with air or either of two liquids L₁ and L₂ having refractive indices n₁and n₂ respectively (n₂> n₁> 1). The lens will diverge a parallel beam of light if it is filled with –

• A. Air and placed in air
• B. Air and immersed in L₁
• C. L₁ and immersed in L₂
• D. L₂ and immersed in L₁

Answer 1

Answer: (D)

L₂ and immersed in L₁

Answer 2

= $\left( \frac { \mathrm { n } _ { 2 } } { \mathrm { n } _ { 1 } } - 1 \right)$ $\left( \frac { 1 } { \mathrm { R } _ { 1 } } - \frac { 1 } { \mathrm { R } _ { 2 } } \right)$ where n₂ and n₁are the refractive index of the material of the lens and of the surrounding respectively. For a concave lens $\left( \frac { 1 } { \mathrm { R } _ { 1 } } - \frac { 1 } { \mathrm { R } _ { 2 } } \right)$ is always negative

Hence f is negative only when n₂> n₁

option D is correct.