First slide

The optical instruments

Question

A man can see only between 75 cm and 200 cm. The power of lens to correct the near point will be

Moderate

A

$+ \frac{8}{3} D$
B

+ 3 D
C

– 3 D
D

$- \frac{8}{3} D$

Solution

For improving near point, convex lens is required and for this convex lens
$u = - 25 cm, v = - 75 cm ∴ \frac{1}{f} = \frac{1}{- 75} - \frac{1}{- 25} \Rightarrow f = \frac{75}{2} cm So power P = \frac{100}{f} = \frac{100}{75 / 2} = + \frac{8}{3} D$

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