> Lenses bend EM waves proportionately to their frequency

Is this exact? I was under the impression that it's a linear approximation that's generally good enough for optical component glasses over the range of visual wavelengths.

(I always found it a bit frustrating that in my Mechanical Engineering undergraduate classes, they almost always introduced linear approximations without any discussion about the conditions under which the approximations held. Sometimes, they didn't even mention that the linearization was an approximation.)