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?

  • $\begingroup$ chromatic aberration: en.wikipedia.org/wiki/Chromatic_aberration $\endgroup$ – hyportnex Feb 6 '17 at 15:35
  • $\begingroup$ It takes a reasonable amount of effort to make a good achromatic lens. For simple optics exercises it usually doesn't matter. For real work it can matter a lot. $\endgroup$ – Jon Custer Feb 6 '17 at 15:39

The reason why the focal length of a lens depends on the wavelength is the same as the reason why a glass prism can separate the wavelength of light into a rainbow: the refractive index ($\mu$ in your question, I presume) depends on the wavelength of the light. This is called dispersion.


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