I was surprised to see my textbook having the following: "The focal length of a lens does not change when Red light is replaced by Blue light"- as an incorrect statement.
Why would the focal length change? From my knowledge it only depends on $\mu $ (refractive index of the material of lens and also of the medium) and the radius of curvature of lens following the lens maker equation? So how would that change on changing the light frequency ?
I realize that $\mu $ would be different for different colors i.e red , violet, green and so on. But if asked in the question about "lens" then won't we consider a single spectrum only?