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प्रश्न
State the nature of the lens and the focal length if its power is +4D.
संख्यात्मक
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उत्तर
(i) As the power is +4D, therefore the lens is convex.
\[ \text{(ii) Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]
\[ \Rightarrow 4 = \frac{100}{f \text{(in cm)}} \]
\[ \Rightarrow f \text{(in cm)} = \frac{100}{4} \]
\[ \therefore \ f = + \ 25 \ \text{cm} \]
Thus, the focal length of the lens is 25 cm.
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