question_answer

The image of an illuminated square is obtained on screen with the help of a converging lens. The distance of the square of the square from the lens is 40 cm. The area of the image is 9 times that of the square. The focal length of the lens is: [JEE ONLINE 22-04-2013]

A)             36 cm

B) 27 cm

C)             60 cm

D) 30 cm

Correct Answer: D

Solution :

[d] If side of object square

and side of image square

From question,

or

i.e., magnification

From formula,

or,