The focal length of the combination of lens is given by 1/F = 1/f1 + 1/f2
If µ and µ’ are mean refractive indices for crown and flint glasses, respectively, then,
The dispersive powers ω and ω' for crown and flint glass, respectively, are
To have an achromatic combination,
The achromatic doublet requires a convex lens of focal length 37.5 cm made of crown glass and a concave lens of focal length 60 cm made of flint glass.
