Question bank/JEE Main/Physics

A convex lens forms inverted image of a real object on a fixed
screen. The size of image is 12 cm. When lens is displaced 20 cm along
principle axis it again forms a real image of size 3 cm on the screen.
Focal length (in cm) of the lens is 40/n. Find n. (Assume image
formation only by paraxial rays)

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since [IMAGE] For first position of the lens
[IMAGE] From displacement method
Using
$f = \frac{D^{2} - \Delta d^{2}}{4D} = \frac{60^{2} - 20^{2}}{4 \times 60} = \frac{40}{3}\text{cm}$

Source: Grass-G / P-14 Physics paper 7 Nov.docx