First slide

Dispersion

Question

The dispersive powers of crown and flint glasses are 0.02 and 0.04 respectively. In an achromatic combination of lenses the focal length of flint glass lens is 40 cm. The focal length of crown glass lens will be

Moderate

A

– 20 cm
B

+ 20 cm
C

– 10 cm
D

+ 10 cm

Solution

$By using \frac{ω_{1}}{f_{1}} + \frac{ω_{2}}{f_{2}} = 0 \Rightarrow \frac{0 .02}{f_{1}} + \frac{0 .04}{40} = 0 f_{1} = - 20 cm$

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