I was surprised to see my text saying that the " focal length of lens does not change when red light is replaced by Blue light"- as incorrect statement
what I think is that why would the focal length change doesn't it depends on $\mu $ and radius of curvature of lens follwing the lens maker equation? So how would that change on changing light frequency ?
Though I also think that although $\mu $ would be different for different red , violet, green and so on. But if asked about "lens" then don't we consider a single spectrum only?