Assertion : Electromagnetic waves with frequencies smaller than the critical frequency of ionosphere are used for communication using sky wave propagation. Reason : The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.

Step-by-step solution

If the frequencies of the waves are less than that of the critical frequency. They will be reflected back as the ionosphere has refractive index less than free space. This is the principle of sky wave propagation. The statement given in the Reason that the refractive index becomes high is correct. But the Reason given is not true for the assertion.