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प्रश्न
The focal length of the lens of a myopic person is 40 cm. What is the power of the lens?
संख्यात्मक
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उत्तर
As the person is myopic (short sighted), the lens of his glasses is concave.
Focal length of the concave lens (f) = −40 cm [f for concave lens is −ive]
\[ \text{Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]
\[ \Rightarrow \mathrm{P} = \frac{100}{-40} \]
\[ \Rightarrow \mathrm{P} = - \ 2.5 \ \text{D} \]
Minus sign indicates that the lens is concave in nature.
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