In a displacement method, there are two positions of a lens for which we get a clear image. First position of the lens is at 40cm from object and second is at 80cm, the focal length of the lens is?

first position u=-40 cm. let the screen be at d from the lens 1d+140=1f40+d40d=1f40f+df=40d40f40-f=dfor second position u=-80 cm ,v=d-401d-40-1-80=1f140f40-f-40-1-80=1f1f-20=4ff=4f-80f=803 cm Regards

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