A student can distinctly see the object upto a distance 15 cm. He wants to see the black board at a distance of 3 m. Focal length and power of lens used respectively will be
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a
- 4.8 cm, - 3.3 D
b
- 5.8 cm, - 4.3 D
c
- 7.5 cm, - 6.3 D
d
- 15.8 cm, - 6.3 D
answer is D.
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Detailed Solution
v=−15cm,u=−300cm From lens formula 1f=1v−1u ⇒1f=1−15−1−300=−19300⇒f=−30019=−15.8cm and power P=100fcm=−100×19300=−6.33D.