Group velocity and pulse-envelope limits
For a narrow-band wave packet on a smooth real dispersion branch, its leading envelope velocity is v_g=dω/dk at the carrier, while phase velocity is ω/k.
Key knowledge
For a narrow-band wave packet on a smooth real dispersion branch, its leading envelope velocity is v_g=dω/dk at the carrier, while phase velocity is ω/k. Higher dispersion derivatives broaden or distort the packet; group velocity need not equal signal-front velocity in strongly dispersive or absorbing media.
Values and equations
v_g=dω/dk; v_phase=ω/k
- v_g,v_phase
- Group/phase velocity m s⁻¹
- ω
- Angular frequency rad s⁻¹
- k
- Wavenumber m⁻¹
Conditions and scope
- Narrow spectral width, smooth approximately real dispersion and negligible reshaping across the chosen propagation range for the leading envelope interpretation.
- Near absorption/resonance or with broad spectra, assigning one derivative velocity to the entire signal can be misleading.
Sources · 1
References
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Evidence and review
Classical Electromagnetism (2014): Group velocity: Equations 918–919 and stationary-phase interpretation; excludes imaginary-value precursor approximations from the real-branch envelope statement.
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Entry details
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- physics-group-velocity
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