First slide

The optical instruments

Question

A person's near point is 50 cm and his far point is 3 m. Power of the lenses he requires for
(i) reading and
(ii) for seeing distant stars are

Moderate

A

– 2 D and 0.33 D
B

2 D and – 0.33 D
C

– 2 D and 3 D
D

2 D and – 3 D

Solution

For correcting the near point, required focal length
$f = \frac{50 \times 25}{(50 - 25)} = 50 cm So power P = \frac{100}{50} = + 2 D For correcting the far point, required focal length f = - (defected far point) = - 3 m ∴ P = - \frac{1}{3} D = - 0 .33 D$

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