Define the term critical frequency in relation to sky wave propagation of electromagnetic waves.

Explanation

Critical frequency is the highest frequency of
the radiowaves which when sent straight
towards the given layer of ionosphere gets
reflected from ionosphere and returns to the
earth. It is given by
`v_c = 9(N_(max))^(1//2)`
where `N_(max)` is the maximum number density of
electrons in the given layer of ionosphere.