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ICSE Class 10 Physics 2018 Paper

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The power of a lens is −5D-5 D.
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Question : 11 of 78
Marks: +1, -0
Find its focal length.
Solution:  
Given:
Power of lens =−5D=-5 D
∴   focal length     =  100 Power    (in cm)   \therefore \; \text{ focal length } \;\; = \; \frac{100}{\text{ Power }} \; \text{ (in cm) } \;
  =  100−5=−20 cm\; = \; \frac{100}{-5} = -20 \text{ cm}
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