Question

Two lenses of focal length 20 cm and -40 cm are held in contact. The image of an object at infinity will be formed by the combination at :

• A. 10 cm
• B. 20 cm
• C. 40 cm
• D. Infinity

Answer 1

Answer: (C)

40 cm

Answer 2

: Equivalent focal length

Here, and

$\frac { 1 } { \mathrm { f } } = \frac { 1 } { 20 } - \frac { 1 } { 40 } = \frac { 2 - 1 } { 40 } \Rightarrow \mathrm { f } = 40 \mathrm {~cm}$

$\therefore \frac { 1 } { \mathrm { f } } = \frac { 1 } { \mathrm { v } } - \frac { 1 } { \mathrm { u } }$

Here, f = 40cm,u = $\infty$

$\frac { 1 } { \mathrm { f } } = \frac { 1 } { \mathrm { v } } - \frac { 1 } { \infty } \Rightarrow \mathrm { v } = \mathrm { f } = 40 \mathrm {~cm}$