Question

A student focussed the image of a candle flame on a white screen using a lens. He noted down the position of the candle screen and the lens as mentioned below:
Position of candle = 12.0 cm
Position of lens = 50.0 cm
Position of the screen = 88.0 cm

(ii) What is the focal length of the lens used?

• A. 12 cm
• B. 19 cm
• C. 22 cm
• D. 25 cm

Answer 1

The correct option is B 19 cm

Given:
Position of candle = 12.0 cm
Position of lens = 50.0 cm
Position of the screen = 88.0 cm

Measuring all the distances from the pole, we get:
Object distance, $u=12-50=-38cm$
Image distance, $v=88-50=+38cm$

Using lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{+38}-\frac{1}{-38}=\frac{1}{f}$
$f=+19cm$

Focal length of the lens is positive. So, it should be a convex lens.