Given: 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. The ratio of dispersive powers of flint glass to crown glass is 1.5.
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