Question

how does focal length of a lens depends on the wavelength of light passed through?according to me the focal length should not change as focal length is a physical quantity of a lens so it should not change.

Answer 1

For a spherical mirror the focal length remains unaffected with all three parameters; wavelength, refractive index and frequency.

The focal length of the lens varies directly with the wavelength of the light used and as wavelength is inversely related to refractive index, the focal length also varies inversely with it. This is the principal cause of the phenomena of chromatic aberration. The frequency of light has no effect on the focal length.

So, focal length in creases with wavelength but decreases with refractive index and vice versa.