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The focal length of a convex lens made of flint glass is 15cm. To remove its chromatic aberration, it is placed in contact with a concave lens made of crown glass. Find the focal length of the concave lens. The ratio of dispersive powers of flint glass to crown glass is 1.5.

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Solution

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To find the focal length of the concave lens

Solution:

Let focal length of the convex lens be f1 and focal length of the concave lens be f2 and ratio of dispersice power of the flint glass is $\dfrac {\omega_2}{\omega_1}=1.5$

For achromatic combination,

f1f2=−ω2ω1⟹15f2=−1.5⟹f2=−10cm

Hence the focal length of the concave lens is -10cm

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