(i) Sky wave propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz. Sky wave can travel very long distances and can even travel round the earth.
(ii) Refractive index of ionosphere (x) is less than that of the layers of atmosphere beneath it (x0). As we go high, the electron density decreases, x' decrease. The refraction occurs till it reaches critical angle and afterwards it reflects electromagnetic wave back on earth's surface.
The critical frequency is given by
As 30 MHz > fc> 5 MHz,
When frequency is greater then fc, it crosses ionosphere and never reflects back on the surface of earth. The ranges of fe up to few MHz sets an upper limit to frequency of waves used in this mode.