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 formula1f=1v−1u⇒1f=1−15−1−300=−19300⇒f=−30019=−15.8cmand power P=100fcm=−100×19300=−6.33 D