First slide

The optical instruments

Question

A person who can see things most clearly at a distance of 10 cm. Requires spectacles to enable to him to see clearly things at a distance of 30 cm. What should be the focal length of the spectacles

Moderate

A

15 cm (Concave)
B

15 cm (Convex)
C

10 cm
D

0

Solution

$For lens u = wants to see = - 30 cm and v = can see = - 10 cm ∴ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{- 10} - \frac{1}{(- 30)} \Rightarrow f = - 15 cm$

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