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Below the critical frequency a wave sent vertically into the ionosphere is

  • A.transmitted through into space
  • B.converted into a circularly polarised wave
  • C.reflected back to earth
  • D.absorbed entirely within the ionised layer

Answer: C

The effective permittivity falls to zero at that frequency, the wave can go no further and it returns. Above it the wave passes through, which is why satellite links use frequencies well above the critical value and broadcast services below it.

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