Question

Two lenses of power +10D and -5 D are placed in contact. Where should an object be held from the lens, so as to obtain a virtual image of magnification 2?

• A. 5 cm
• B. -5 m
• C. 10 cm
• D. -10 cm

Answer 1

Answer: (D)

-10 cm

Answer 2

: Here, and $P _ { 2 } = - 5 D$

Therefore, power of the combined lens is

Now, magnification, $\mathrm { m } = \frac { \mathrm { f } } { \mathrm { u } + \mathrm { f } }$

Here, m = 2 and $\mathrm { f } = \frac { 1 } { \mathrm { P } } = \frac { 1 } { 5 } = 0.2 \mathrm {~m} = 20 \mathrm {~cm}$

$\therefore 2 = \frac { 20 } { u + 20 } \Rightarrow u + 20 = 10$

$\Rightarrow \mathrm { u } = - 10 \mathrm {~cm}$