Assertion : Electromagnetic waves with frequencies smaller than the critical frequencies 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 critical frequency.

Step-by-step solution

The highest frequency above which the ionosphere no longer returns the sky wave back to earth when transmitted in vertical direction is called critical frequency, so for sky wave communication, frequency of electromagnetic wave should be less than the critical frequency. Refractive index of a medium is given by =A+ B ^2 + C ^4 +