I'm revising for a uni exam with past exam papers, and have gotten stuck on the details of dispersion. The two exam questions prompting this are a) What is the physical reason why the index of refraction for blue light is bigger than that of red light? and b) Explain how dispersion makes a single lens give a blurry image. I know that dispersion of light links the index of refraction of a medium to the frequency and/or wavelength of the light passing through it. I'm stuck with the and/or part; when I'm answering questions like the ones above, (especially for a)), do I explain how different colours of light have different frequencies, or different wavelengths (even though both options are true)? I know that the frequency of light doesn't change, no matter the medium the light travels through, but the wavelength of the light does change, depending on medium. So would that mean (as the actual velocity of light appears to change when the light enters a different medium) that its index of refraction would really be dependant on wavelength rather than frequency? And finally, regarding chromatic aberration, why is focal length of a lens dependent on refraction? Is there a (simple-ish) formula showing this? This is something I've just thought about as I was typing, so it's slightly unrelated, but I figured this was the right place to ask. Relevant equations are n = c/v and v = fλ, and my attempts at solutions are a) Dispersion - because as the frequency of blue light is higher than that of red light, no matter the wavelengths of each of them. and b) Lenses have different refractive indexes for different frequencies of light, which results in chromatic aberration as the focal length of a lens is dependant on its refractive index, which means that the different indexes of refraction for different colours of light will result in the different colours not focusing at the same point, thus creating a blurred image. In typing out this question, things have begun to fall into place, but further information and/or insight would be welcomed. Thank you very much for any and all help!