First slide

The optical instruments

Question

A man can see clearly up to 3 metres. Prescribe a lens for his spectacles so that he can see clearly up to 12 metres.

Moderate

A

$- \frac{3}{4} D$
B

3 D
C

$- \frac{1}{4} D$
D

– 4 D

Solution

$For lens u = wants to see = - 12 cm v = can see = - 3 m ∴ P = \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow P = \frac{1}{- 3} - \frac{1}{(- 12)} = - \frac{1}{4} D$

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