Question

a convex lens forms an image of an object on a screen 30cm from the lens. when the lens is moved 90cm towards the object,the image is again formed on the screen. Then the focal length of the lens is:-
1) 13cm approximately
2) 23cm approximately
3) 33cm approximately
4) 40cm approximately

Answer 1

we know

1/f = 1/v + 1/u
where

u - object distance
v - image distance

now
1/f
= 1/30 + 1/90
=(3 + 1)/90[90 is LCM]
=4/90
=(2 x 2) / (45 x 2)
one pair of 2 will get cancelled
=2/45

now f = 45/2= 22.5cm

23 cm approximately

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the correct answer is- (2)

Answer 2

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