JEE MAIN - Physics (2024 - 30th January Evening Shift - No. 29)
In an experiment to measure the focal length $$(f)$$ of a convex lens, the magnitude of 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 figure.
The focal length of the lens is ________ $$\mathrm{cm}$$.
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Answer
20
Explanation
$$\begin{aligned} & \frac{1}{f+20}-\frac{1}{-(f+20)}=\frac{1}{f} \\ & \frac{2}{f+20}=\frac{1}{f} \quad f=20 \mathrm{~cm} \end{aligned}$$
Or $$\mathrm{x}_1 \mathrm{x}_2=\mathrm{f}^2$$ gives $$\mathrm{f}=20 \mathrm{~cm}$$
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