First slide

The optical instruments

Question

A person is suffering from myopic defect. He is able to see clear objects placed at 15 cm. What type and of what focal length of lens he should use to see clearly the object placed 60 cm away?

Moderate

A

Concave lens of 20 cm focal length
B

Convex lens of 20 cm focal length
C

Concave lens of 12 cm focal length
D

Convex lens of 12 cm focal length

Solution

For viewing far objects, concave lenses are used and for concave lens
$u = wants to see = - 60 cm; v = can see = - 15 cm$
$so from \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow f = - 20 cm .$

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