## Measure Phase velocity/group velocity of EM-wave

Hey folks,

some weeks ago we had an trial-exam and one of the questions there was: "How do you measure phase and group velocity."

That question really got me. Having a fastly oscillating wave as carrier and then the envelope. Can someone help me? Is it impossible for the phase, as it is usually a harmonic one?

Would be really great if someone would know the answer,
take care,
spookyfw
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 Recognitions: Gold Member Science Advisor I am assuming you are talking about EM signals. How you do it depends on the frequency range. For RF/Microwave you can use a phase detector, these are either dedicated components that you can buy; or you use a saturated mixer.
 Sorry for this very late reply. First of all: thank you very much for your answer. Yes, I was talking about EM-waves. More about a pulse in the visible region though. I wondered how one would then discriminate and measure phase and group velocity. Using a phase detector sounds reasonable, but how does it work? Would be really grateful if someone would know :).

## Measure Phase velocity/group velocity of EM-wave

Hi spookyfw,

Except in very special circumstances, measurement of phase velocity is not feasible for a number of reasons:

1. The surface that represents the constant phase of the entire wave packet may not necessarily be a plane.

2. That surface may not necessarily travel in the same direction as the wave.

3. The amplitude of the wave on that surface may be extremely small.

4. That surface may lie outside of the causal influence of the wave.

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