speed of all wavelengths of light in visible spectrum is same in air but different in medium(glass).Why?
Your question centers around the nature of ‘free space’ or the vacuum state as defined by EM theory and the nature of dispersion as applied to Electro-magnetic radiation.
All mediums that light passes through other than the defining ‘free space’ of the theory have some wavelength dependence for group and phase velocities. It is just that for all practical purposes we can treat air as a non-dispersive medium because the shift is small when we compare it to other mediums.
The same applies by the way to ‘space’ as we know it. The space surrounding planets, stars, galaxies, etc is not the ‘free space’ of EM theory.
The concept of ‘free space’ is an abstract construct and is used as a defining point. Although we are getting better at approximating it under certain laboratory conditions.
As to why there should be a dispersive relationship at all is due to the concept of permittivity. Permittivity is a measure of a medium’s ability to react to an applied electric field. It defines how easily a medium ‘permits’ the propagation of an electric field and is also related to the electric susceptibility. There is a frequency dependence. You may think of this as how fast the material can react to an applied electric field by polarizing its charges. One would obviously expect the material to show a frequency dependence ( faster changes are harder to follow.)
I hope this helps.
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