Question

A convex lens forms a real image on a screen when an object is placed at 10 cm from the screen. On shifting the lens by 2 cm, a real image is again formed on the screen. Find the focal length of the lens.

• A. $2cm$
• B. $10cm$
• C. $20cm$
• D. $2.4cm$

Answer 1

The correct option is D $2.4cm$

$f=\frac{D^2-{\Delta }^2}{4D}$

this is formula of finding focal length using displacement method.

$f:-$ focal length

$\Delta :-$ displacement of lens done in order to get image again on screen.

$D:-$ distance between object and screen.

$f=\frac{{10}^2-2^2}{4\times 10}$

$f=2.4$ cm