I read that the phase Velocity of an X-Ray is greater than C, however its Signal Velocity is not.

Huh? :bugeye:

If I understand Phase Velocity, it is the rate at which of any point of a wave (the crest, for example) propagates through space.

How can the speed of the crests of a wave exceed the speed of the wave itself?

What am I missing here?

Also, what is Group Velocity and how does it differ from Signal Velocity?

Originally posted by one_raven
I read that the phase Velocity of an X-Ray is greater than C, however its Signal Velocity is not.

Huh? :bugeye:

If I understand Phase Velocity, it is the rate at which of any point of a wave (the crest, for example) propagates through space.

How can the speed of the crests of a wave exceed the speed of the wave itself?

What am I missing here?

Also, what is Group Velocity and how does it differ from Signal Velocity?

group velocity and signal velocity are the same thing. energy is transfered at the group velocity, and this can never exceed the speed of light.

phase velocity is the speed of a single "harmonic" of a wave packet. that portion of the wave that has a constant wave number. it is something of a mathematical tool, actually, it s not really a physically measurable number. therefore it is possible for the phase velocity to exceed the group velocity.

the phase velocity of a wave is ω/k, and the group velocity is dω/dk.