Question

A converging lens of focal length $20cm$ and a diverging lens of focal length $5cm$ are placed coaxially at $d$ distance apart. If a parallel ray of light is incident on the converging lens and it emerges out of the diverging lens as a parallel ray, then the value of $d$ will be:

• A. $30cm$
• B. $25cm$
• C. $15cm$
• D. $50cm$

Answer 1

The correct option is C $15cm$

Given,

Focal length of converging lens, $f_1=20cm$

Focal length of diverging lens,$f_2=-5cm$

$f_{eq}=\frac{f_1{f}_2}{f_1+f_2-d}$

Since there is no deviation,

$f_{eq}=\infty$

$\Rightarrow f_1+f_2-d=0$

$\Rightarrow f_1+f_2=d$

$20-5=d$

$\Rightarrow d=15cm$