Question

A thin glass (refractive index 1.5) lens has optical power of -8 Din air. Its optical power in a liquid medium with refractive index 1.6 will:

• A. 1 D
• B. -1 D
• C. 25 D
• D. -25D

Answer 1

Answer: (A)

1 D

Answer 2

: In air,

$\mathrm { P } = \frac { 1 } { \mathrm { f } } = \left( \frac { \mu _ { \mathrm { g } } } { \mu _ { \mathrm { a } } } - 1 \right) \left( \frac { 1 } { \mathrm { R } _ { 1 } } - \frac { 1 } { \mathrm { R } _ { 2 } } \right)$

In medium

$\frac { \mathrm { P } ^ { \prime } } { \mathrm { P } } = \frac { \left( \frac { \mu _ { \mathrm { g } } } { \mu _ { 1 } } - 1 \right) } { \left( \frac { \mu _ { \mathrm { g } } } { \mu _ { \mathrm { a } } } - 1 \right) } = \frac { \left( \frac { 15 } { 1.6 } - 1 \right) } { \left( \frac { 1.5 } { 1 } - 1 \right) } = \frac { ( - 0.1 / 1.6 ) } { 0.5 }$