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The image of an illuminated square objec...

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 ions ?

A

30 cm

B

36 cm

C

22.5 cm

D

18 cm

Text Solution

Verified by Experts

The correct Answer is:
C
`M=(I)/(O)=(v)/(u) therefore 3=(v)/(u)`
`therefore v=3u=3xx30=90 cm`
For a convex lens, f is positive, u is negative and as it is a real image v is positive.
`therefore (1)/(f)=(1)/(v)-(1)/(u)=(1)/(90)+(1)/(30)=(4)/(90)`
`therefore f=22.5 cm`
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