# I Phase velocity is confusing

1. Sep 6, 2016

### KFC

Hi all,
I have a difficulty to understand the concept of phase velocity of a wave packet. I read some materials online and my text, I didn't find a very straightforward explanation on this concept except the math definition. I also read some threads from this forum, but still have some questions.

From wiki, and I rephrase it as "the phase velocity is how the phase of any frequency component of a wavepacket moves". To me, it implies that all frequency components, if you starring at one particular point of each frequency component, they will move at the same pace? This is confusing because in one optical experiment on dispersion, we know that different color of light moves at different speed inside the prism. So is the travelling speed anything to do with this phase speed? If so, why here they said the phase speed same for all components.

I know that the light travel speed for different color is not same as the group speed since group speed means how fast the envelope travels.

2. Sep 6, 2016

### DuckAmuck

It's the same for all components in a vacuum.

3. Sep 6, 2016

### KFC

Oh, so to briefly summarize it, can I say phase speed is the speed of each frequency component composing the wave. Phase speed could be same or different for each component. If phase speed is different for each components, there will be dispersion.

4. Sep 6, 2016

5. Sep 6, 2016

thanks