First slide

The lens

Question

An achromatic combination of a concave and convex lenses has power $5 D$ . If the power of convex lens is $4 D$ , then the magnitude of focal length of concave lens is

Easy

A

$10 c m$
B

$200 c m$
C

$100 c m$
D

$20 c m$

Solution

$P = P_{1} + P_{2}$
$5 = 4 - P_{2}$
$P_{2} = 5 - 4 = - 1 D$
$f_{2} = \frac{100}{P_{2}} = \frac{100}{- 1} = - 100 c m$

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