Question

A screen is placed at a distance $40 \, cm$ away from an illuminated object. A converging lens is placed between the source and the screen and an attempt is made to form the image of the source on the screen. If no position of the lens could be found, the focal length of the lens

• A. must be less than $10 \, cm$
• B. must be greater than $10 \, cm$
• C. must not be greater than $20 \, cm$
• D. must not be less than $10 \, cm$

Answer 1

Answer: (B)

Distance between object and image or screen is $d=40 \, cm$
If a converging lens makes real image of a real object then
$d\geq 4f$
$f\leq \frac{d}{4}$
$f\leq \frac{40}{4}$
$f\leq 10 \, cm$
This is a condition for obtaining an image. As no image is found therefore focal length must be greater than $10 \, cm$ .