Question

A thin convex lens of refractive index 1.5 has a focal length of 15 cm in air. When the lens is placed in liquid of refractive index 4/3, its focal length will be –

• A. 15 cm
• B. 10 cm
• C. 30 cm
• D. 60 cm

Answer 1

Answer: (D)

60 cm

Answer 2

f '= $\frac { ( \mu - 1 ) \mu ^ { \prime } } { \left( \mu - \mu ^ { \prime } \right) }$ f (given m = $\frac { 3 } { 2 }$; m' = $\frac { 4 } { 3 }$; f = 15cm)

= $\frac { \left( \frac { 3 } { 2 } - 1 \right) \frac { 4 } { 3 } } { \left( \frac { 3 } { 2 } - \frac { 4 } { 3 } \right) } \times 15$ = $\frac { \frac { 1 } { 2 } \times \frac { 4 } { 3 } } { \frac { 1 } { 6 } }$ ×15 = 60 cm

$\mathrm { f } ^ { \prime } = 60 \mathrm {~cm}$