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Wave Optics
Question
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(i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz ? Explain diagrammatically how long distance communication can be achieved by this mode.

(ii) Why is there an upper limit to frequency of waves used in this mode?

Answer

i) Sky-wave propagation is used for long distance communication. Long distance communication is achieved by reflection of radio waves by the ionosphere, back towards the earth. This ionosphere layer acts as a reflector for certain range of frequencies from few MHz to 30 MHz.


ii) There is an upper limit to the frequency of waves used in sky wave propagation because electromagnetic waves of frequencies higher than 30MHz, penetrate the ionosphere and escape whereas waves less than 30 MHz are reflected back to the earth by the ionosphere.

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