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A lens of focal length 20 cm forms the image of a bright object kept 30 cm from it on a screen. If the lens is replaced by another lens, the object has to be moved by 5 cm to obtain a sharp image on the screen. Find the possible values of the focal length of the second lens.

A lens of focal length 20 cm forms the image of a bright object kept 30 cm from it on a screen.
If the lens is replaced by another lens, the object has to be moved by 5 cm to obtain a sharp
image on the screen. Find the possible values of the focal length of the second lens.

Grade:10

1 Answers

sanatan sharma
26 Points
12 years ago

u1 = 20cm

f1 = 15cm

therefore 1/v1 = 1/u1 + 1/f1

1/v1 = 1/20 + 1/15

1/v1 = 7/60

v1 = 60/7

now object is shifted 

two cases arise 1)- object moved away from second lens 2)- object is moved towards the lens

v2 = 60/7

u2 = 35 , 25

therefore the two possible focal lenghts will come out as 420/37cm  and 300/23cm respectively.

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