PHYSICS: speed of visible light in glass

From classical electromagnetic theory developed by Clark Maxwell in the late 19 Century (the Maxwells equations), it has been shown that the speed of electromagnetic waves (EM waves) depends on the electric property (the permitivity) and magnetic property (the permeability) of a medium in which EM waves pass through. The reason being that EM wave travels by changing electric and magnetic fields. These changing electric and magnetic fields will interact with the atomic dipoles of the medium, thus affecting the wave speed.

The higher the permitivity or permeability of a material, the lower is the light speed because of strong interaction of the atomic dipoles with the electric and magnetic fields of the EM wave. Because vacuum has the lowest permitivity and permeability, thus EM wave (and light, which is EM wave in nature) travels fastest in vacuum.

Both permitivity and permeability are frequency dependent. In the visible range, red light (which has the lowest frequency) has the lowest permitivity and permeability among other coloured lights, thus having the highest speed.