Question

A concave lens of glass, refractive index 1.5, has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a –

• A. Convergent lens of focal length 3.5 R
• B. Convergent lens of focal length 3.0 R
• C. Divergentlens of focal length 3.5 R
• D. Divergentlens of focal length 3.0 R

Answer 1

Answer: (A)

Convergent lens of focal length 3.5 R

Answer 2

= $\frac { ( \mu - 1 ) \mu ^ { 1 } } { \left( \mu - \mu ^ { \prime } \right) }$ = $\frac { ( 1.5 - 1 ) \times 1.75 } { ( 1.5 - 1.75 ) }$ = – 3.5

f_a = – R so f_l = + 3.5 R

option A is correct.