Q 12-09-100JEE MainJEE Main 2024 (30 Jan, Shift 2)Medium
In an experiment to measure the focal length $(f)$ of a convex lens, the magnitudes of the object distance $(x)$ and the image distance $(y)$ are measured with reference to the focal point of the lens. The $y$–$x$ plot is shown in the figure. The focal length of the lens is ______ cm.
Numerical value type. Enter your answer.
Answer: 20
With distances measured from the focal points, Newton's lens formula gives $xy = f^2$.
At point $C$ the curve passes through $x = y = 20\ \text{cm}$ (the other plotted points also give $xy = 400\ \text{cm}^2$):
$$f = \sqrt{20\times20} = 20\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics