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. Divergent lens of focal length 3.5 R
• D. Divergent lens of focal length 3.0 R

Answer 1

Answer: (A)

Convergent lens of focal length 3.5 R

Answer 2

$\frac { 1 } { \mathrm { f } } = \left( \frac { 1.5 } { 1.75 } - 1 \right) \left( \frac { 1 } { - \mathrm { R } } - \frac { 1 } { \mathrm { R } } \right)$

f = = + 3.5R, converging