The image of an illuminated square object is obtained on a screen with the help of a converging lens. The distance of the square from the lens is 30 cm. The area of the image is 9 times of the square. What is the focal length of the lens ?

Options

A. 36 cm
B. 27 cm
C. 60 cm
D. 30 cm

Answer

D

Explanation

If side of object square = l
and side of image square = l.
From question, `(l.^(2))/(l)=9 or (l.)/(l)=3`
i.e., magnification m = 3
`u=-40cm`
`v=3xx40=120cm`
`f=?`
`(1)/(120)-(1)/(-40)=(1)/(f) or, (1)/(f)=(1)/(120)+(1)/(40)=(1+3)/(120) therefore f=30cm`