1.

Assertion: Em wave with frequencies more than the critical frequency of ionosphere cannot be used for communication using sky-wave propagation. Reason: The refractive index of the ionosphere becomes very high for frequencies higher than the crtical frequency.

Answer»

If both the assertion and reason are true and reason is a true explantion of the assertion.
If both the assertion and reason are true but thereason is not true the correct explantion of the assertion.
If the assertion is true but reason false
If both the assertion and reason are false.

Solution :The sky waves are the radiowave of frequency range between `2MHz` and `30 MHz`. The ionosphere reflects these radiowaves back to earthduring their propagation through atmosphere. The refractive index of ionosphere is less thanits free SPACE value. That is, it behaves as a rarer medium. Threrefore, the wave will turn away from the normal when it enters the ionosphere. As we go DEEP into the ionosphere (electron density `N` is large), the refractive index keeps on decreasing.

The refraction or bending of the beam will continue till it reaches ctritical ANGLE after which it is will be reflacted back. If the frequency is too high, then after a CERTAIN value, the electron density `N` may never be so high as to produce enough bending for attainment of critical angle or condition of reflection. This is called ctritical frequency. For frequencies higher than this value, the refractive index of the ionosphere becomes very high, so they cross the ionosphere and do not return back to the earth. Hence, the option (a) is true.


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