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An eye specialist prescribes spectacles having combination of convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm. The power of this lens combination in diapters is

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+1.5

-1.5

+6.67

-6.67

answer is B.

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Detailed Solution

$f_{1} = 40 cm f_{2} = 25 cm$

Power $P = \frac{1}{f}$

$\begin{array}{l} P_{1} = \frac{1}{0 .4} = \frac{10}{4} = 2 .5 D \Rightarrow P_{1} = 2 .5 D \\ P_{2} = \frac{1}{0 .25} = \frac{100}{25} = 4 D \Rightarrow P_{2} = - 4 D \\ P = P_{1} + P_{2} = 2 .5 - 4 = - 1 .5 D \end{array}$

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