Having been working on a wave packet that is made up of lots of different sinusoidal waves together, the result is a set of inner waves inside an outer envelope. The envelope moves at a different speed to the inner waves - hence the distinction between the outer group velocity and the phase velocity of the inner waves. I like the last animation on this page
https://en.wikipedia.org/wiki/Phase_velocity This shows the shorter wavelength waves (in blue) travelling with a higher phase velocity then the longer wavelength ones. I hadn't realised that the different waves would go at different speeds - but I suppose that the different colours of light go different speeds in glass.
