Question 30

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 ____ cm.

Correct Answer: 20

According to Newton's lens formula: $$xy = f^2$$

From the given $$y-x$$ hyperbolic plot,

$$\text{At point C: } x = 20\text{ cm}, \quad y = 20\text{ cm}$$

$$\implies 20 \times 20 = f^2$$

$$\implies f^2 = 400 \implies f = 20\text{ cm}$$

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