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The real iamge of a square hole in scree...

The real iamge of a square hole in screen, illuminated be light is obtained on another screen with the help of a converging lens. This distance of the hole from the lens is 20 cm. the are of the image is nine times that of the hole. What is the focal length of the lens ?

A

30 cm

B

25 cm

C

20 cm

D

15 cm

Text Solution

Verified by Experts

The correct Answer is:
D
In this case, the area of the image = 9 times the area magnification = 9
and linear magnification `sqrt(9) =3`
`therefore (v)/(u)=3 " or " v=3u " as " u=-20 cm and v=60 cm`
`therefore (1)/(v)-(1)/(u)=(1)/(f) therefore (1)/(60)+(1)/(20)=(1)/(f)`
`therefore (1+3)/(60)=(1)/(f) therefore f=15 cm`
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