- #1
Glenn G
- 113
- 12
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 traveling at vacuum speeds the pulse should have taken 0.2ns but actually arrived 62ns before it would have done traveling in a vacuum so in a sense traveling 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.
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.
