The distance between an object and the screen | vedclass

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(B) The displacement method for finding the focal length of a convex lens states that for a fixed distance $D$ between the object and the screen,if th

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$ \approx 5 \ D$

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(B) The displacement method for finding the focal length of a convex lens states that for a fixed distance $D$ between the object and the screen,if the lens is placed at two positions separated by distance $x$,the focal length $f$ is given by $f = \frac{D^2 - x^2}{4D}$.Given: $D = 100 \ cm$ and $x = 40 \ cm$.Substituting these values: $f = \frac{(100)^2 - (40)^2}{4 	imes 100} = \frac{10000 - 1600}{400} = \frac{8400}{400} = 21 \ cm$.The power of the lens $P$ is given by $P = \frac{100}{f(cm)} = \frac{100}{21} \approx 4.76 \ D$.Rounding to the nearest integer,the power is approximately $5 \ D$.

The distance between an object and the screen is $100 \ cm$. $A$ lens produces an image on the screen when placed at either of the positions $40 \ cm$ apart. The power of the lens is

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