Q 12-09-031NEETJEE MainMedium
The distance between an object and a screen is $90$ cm. A convex lens placed between them gives a sharp image on the screen for two positions of the lens, $30$ cm apart. The focal length of the lens is
Answer: (B) $20$ cm
Displacement method: $f = \dfrac{D^2 - d^2}{4D} = \dfrac{8100 - 900}{360} = 20$ cm.
Solution by Sreeraj P, M.Sc Physics