# A screen is kept at a distance of 1 m from the object. A converging lens between the object and screen when placed at any positions which are 60cm apart forms a sharp image of the object on the screen, then focal length of lens is:

This problem is based on 'Displacement method' which is commonly used to determine the focal length of a converging lens.
a. As discussed earlier,
$f=\frac{D^{2}-x^{2}}{4 D}$
where D is distance between the object and the screen.
x is the distance between the two positions of lens for which sharp images of the given object are obtained on the screen.
Here, D=1m=100cm
x=60 cm
$\begin{array}{l} f=\frac{(100)^{2}-(60)^{2}}{4 \times 100} \\ f=16 \mathrm{cm} \end{array}$

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