I read an article with this with great interest but also puzzlement. It talked about sending a pulse of light through a section of laser excited Caesium atoms (6cm I think) if travelling at vacuum speeds the pulse should have taken 0.2ns but actually arrived 62ns before it would have done travelling in a vacuum so in a sense travelling at -300x the speed of light in a vacuum or leaving the gas cell before it arrived??? This completely baffles me! It also went on to say about the difference between group and phase velocities and that in a vacuum these are the same but in a dispersive medium not, it did try and explain group and phase velocity by talking about ripples on a pond and that phase velocity is like observing an individual ripple racing along whereas group velocity is like other ripples as a whole moving at what seems a slower speed. I didn't get this analogy as if wavelength stays the same then surely all the ripples individually or as a group are all moving at the same speed. Any learned insight please on any of this greatly appreciated Glenn.