A convex lens forms a real image on a screen when object is placed at 10 cm from the screen. on shifting the lens by 2cm, a real image is again formed on the screen. find the focal length ​

Answers (1)

Using the formula of finding focal length using the displacement method.

i.e f=\frac{D^{2}-\Delta^{2}}{4 D}$

where
f=focal length
 \Delta =displacement of lens done in order to get the image again on screen.
D=distance between object and screen.

we get
f=\frac{10^{2}-2^{2}}{4 \times 10}\\ \Rightarrow f=2.4 \mathrm{cm}

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