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प्रश्न
A lens which forms a real image has a focal length 8 cm. Calculate its power.
संख्यात्मक
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उत्तर
The lens forms a real image, therefore it is a convex lens.
Focal length of the lens (f) = +8 cm [f for convex lens is +ive]
\[ \text{Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]
\[ \Rightarrow \mathrm{P} = \frac{100}{8} \]
\[ \Rightarrow \mathrm{P} = + \ 12.5 \ \text{D} \]
Thus, power is +12.5 D. The positive sign shows that lens is convex.
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