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प्रश्न
Calculate the focal length of a lens of power –2.75 D.
संख्यात्मक
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उत्तर
Power −2.75 D indicates that lens is concave.
\[ \text{Applying, } \mathrm{P} = \frac{100}{f \text{(in cm)}} \]
\[ \Rightarrow - \ 2.75 = \frac{100}{f \text{(in cm)}} \]
\[ \Rightarrow f = -\frac{100}{2.75} \]
\[ \therefore \ f = - \ 36.36 \ \text{cm} \]
Thus, the focal length of the lens is −36.36 cm.
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