An achromatic lens-doublet is formed by placing in contact a convex lens of focal length `20 cm` and a concave lens of focal length `30 cm.` The dispersive power of the material of the convex lens is `0.8.` Determine the dispersive power of the material of the concave lens. Calculate the focal length of the lens-doublet.

Answer

A, B, C

Explanation

(a) `(omega_1)/(f_1)+(omega_2)/(f_2)=0`
`:. omega_2=(-f_2/f_1)omega_1`
`=-((-30))/((+20))(0.18)=0.27`
(b) `1/F=1/(f_1)+1/(f_2)=1/20-1/30`
or `F=+60 cm`