First slide

The optical instruments

Question

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

Moderate

A

$- 4.8 cm, - 3.3 D$
B

$- 5.8 cm, - 4.3 D$
C

$- 7.5 cm, - 6.3 D$
D

$- 15.8 cm, - 6.3 D$

Solution

$v = - 15 cm, u = - 300 cm From lens formula \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{- 15} - \frac{1}{- 300} = \frac{- 19}{300} \Rightarrow f = \frac{- 300}{19} = - 15 .8 cm and power P = \frac{100}{f} cm = \frac{- 100 \times 19}{300} = - 6 .33 D .$

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