Q.Define critical frequency.

Two limits decide whether a sky wave beamed into the ionosphere comes back to earth or is lost into space: the critical frequency and the critical angle.

The critical frequency ($f_c$) is a property of each ionospheric layer. It is the highest frequency of a beamed radio wave that the layer will still refract back to the earth. Waves below $f_c$ are returned; waves above it pass straight through the layer and escape. For the F₂ layer, for example, the critical frequency lies roughly in the range 5 MHz to 12 MHz. This is why only a limited band of frequencies is usable for sky-wave links. …