The sky waves are the radiowave of frequency range between and . The ionosphere reflects these radiowaves back to earth during their propagation through atmosphere. The refractive index of ionosphere is less than its free space value. That is, it behaves as a rarer medium. Therefore, the wave will turn away from the normal when it enters the ionosphere. As we go deep into the ionosphere (electron density is large), the refractive index keeps on decreasing.

The refraction or bending of the beam will continue till it reaches critical angle after which it will be reflected back. If the frequency is too high, then after a certain value, the electron density may never be so high as to produce enough bending for attainment of critical angle or condition of reflection. This is called critical frequency. For frequencies higher than this value, the refractive index of the ionosphere becomes very high, so they cross the ionosphere and do not return back to the earth.