First slide

The lens

Question

The dispersive powers of glasses of lenses used in an achromatic pair are in the ratio 5 : 3. If the focal length of the concave lens is 15 cm then the nature and focal length of the other lens would be

Moderate

A

Convex, 9 cm
B

Concave, 9 cm
C

Convex, 25 cm
D

Concave, 25 cm

Solution

$\frac{ω_{1}}{ω_{2}} = - \frac{f_{1}}{f_{2}} \Rightarrow \frac{5}{3} = \frac{- (- 15)}{f_{2}} \Rightarrow f_{2} = 9 cm$

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