A screen is placed $$90 cm$$ from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by $$20 cm$$. Determine the focal length of the lens.
Distance between the screen and the object $$D=90cm$$.
Distance between two location of convex lens, $$d=20cm$$
The focal length of the lens is given by:
$$f=\dfrac{D^2-d^2}{4D}$$
$$\Rightarrow\dfrac{(90)^2-(20)^2}{4\times90}$$
$$\Rightarrow21.39cm$$
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