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An evanescent field beyond a totally internally reflecting boundary

  • A.carries half the incident power into the second medium
  • B.is a standing wave with nodes a wavelength apart
  • C.decays exponentially and carries no net power across
  • D.propagates into the second medium with reduced amplitude

Answer: C

The field extends a fraction of a wavelength into the second medium but its phase does not advance there, so no net energy crosses. Bringing a second surface within that depth lets energy tunnel across, which is the principle of the optical coupler.

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