Question

A lens forms a real image on a screen placed at a distance $100$ cm from the object if the lens is moved by $20$ cm towards the screen, another image is formed on the screen. The focal length of the lens is

• A. $6cm$
• B. $12cm$
• C. $18cm$
• D. $24cm$

Answer 1

The correct option is D $24cm$

$f=\frac{(D^2-x^2)}{4D}$

Here, $D=100cm,$ and $x=20cm$

$\therefore f=\frac{9(100{)}^2-(20{)}^2}{4\times 100}$

$=\frac{10000-400}{400}$

$=24cm$