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True or false: the phase velocity can exceed the speed of light in vacuum.

  • A.False, except in a medium of relative permittivity below one
  • B.True, and information can then be sent faster than light
  • C.True, in a waveguide or a dispersive medium
  • D.False, nothing being able to exceed the speed of light

Answer: C

The phase velocity is the speed of a point of constant phase and carries no information, so no principle forbids it exceeding c, and in a waveguide it does. The group velocity, which carries the energy, remains below c.

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