Q.Define critical angle.

There is a largest launch angle at which a wave of a given frequency is still bent back to earth by the ionosphere. …

[!TLDR]

The critical angle is the largest angle of incidence at which a radio wave is still returned to earth by the ionosphere.

For a given usable frequency, whether a sky wave is bent back to earth depends on the angle at which it enters the ionosphere. There is a maximum angle of incidence, called the critical angle ($\theta_c$), up to which the wave is refracted enough to return to the earth. Waves beamed at angles greater than the critical angle penetrate the ionospheric layers and escape into outer space instead of retu …