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Q.
$n$ the displacement method, the distance between the object and screen is$ 70 \,cm$ and focal length of the lens is$ 16\, cm$. The separation of the magnified and diminished image positions of the lens is
Solution:
$D=$ Distance of image from object; distance between two lens positions $=d$ and $f=$ focal length of convex lens.
$ d =\sqrt{D^{2}-4 f D} $
$=\sqrt{70^{2}-4 \times 16 \times 70} $
$=\sqrt{420}=20.5 \,cm $