First slide

The optical instruments

Question

A man suffering from myopia can read a book placed at 10 cm distance. For reading the book at a distance of 60 cm with relaxed vision, focal length of the lens required will be

Moderate

A

45 cm
B

– 20 cm
C

– 12 cm
D

30 cm

Solution

For lens u = want’s to see = -60 cm
v = can see = -10 cm
$∴ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{- 10} - \frac{1}{(- 60)} \Rightarrow f = - 12 cm$

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