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(D) Given: Object distance $u = -10 \ cm$, Image distance $v = +20 \ cm$ (since the image is behind the lens, it is real).Using the lens formula: $\fr

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$15$

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(D) Given: Object distance $u = -10 \ cm$, Image distance $v = +20 \ cm$ (since the image is behind the lens, it is real).Using the lens formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$.Substituting the values: $\frac{1}{f} = \frac{1}{20} - \frac{1}{-10} = \frac{1}{20} + \frac{1}{10} = \frac{1+2}{20} = \frac{3}{20} \ cm^{-1}$.Thus, the focal length $f = \frac{20}{3} \ cm$.The power of the lens $P$ in dioptres is given by $P = \frac{100}{f (in \ cm)}$.$P = \frac{100}{20/3} = \frac{100 	imes 3}{20} = 5 	imes 3 = +15 \ D$.

An object placed $10 \ cm$ in front of a lens has an image $20 \ cm$ behind the lens. What is the power of the lens (in dioptres)?

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