some problems about group velocity and phase velocity

[narcisuss]

This is my first post, I think this is a stupid question, but I just can's understand.

And I have read these two concepts on WIKI.

first, I cannot understand the example that group velocity is larger than C(light speed)

as in this website shows us:

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html

secondly, some says that it is the phase velocity can lager than C, some says the group velocity and only it can larger than C, but at that time, the latter one cannot represent any information or energy.

I cannot understand which one is true and why the group velocity cannot carry any information and energy when it > C.

Thanks a lot.

 

[ivar4ever]

Please visit these links. it is clearly explained there.

https://www.mathpages.com/home/kmath210/kmath210.htm
**broken link removed**
**broken link removed**

they are useful in the posted order (acc to me).

Hope this helps