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Assertion: the group velocity in a waveguide is less than the speed of light. Reason: the phase velocity in a waveguide exceeds the speed of light.

  • A.the assertion is true but the reason given for it is false
  • B.the assertion is false although the reason offered is a true statement in itself
  • C.both the assertion and the reason are true and the reason explains the assertion
  • D.both the assertion and the reason are true but the reason does not explain the assertion

Answer: C

In a simple waveguide the product of the two velocities is the square of the free space speed, so one exceeding it forces the other below. The reason therefore does account for the assertion, and relativity constrains only the group velocity.

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