CBSE Class 12-science Answered

In ionosphere refractive index goes on decreasing with the increase in height, so the layer of ionosphere acts as rarer medium with height for electromagnetic wave. Due to it, the incident beam of electromagnetic waves suffers refraction in a layer of ionosphere and is gradually bent farther and farther away from the normal in a layer of ionosphere.The process continues till it reaches upto the critical angle, after it will be reflected back.(i.e sky waves are traveling from denser to rarer medium and are totally internally reflected back at that height where angle of incidence just exceeds the critical angle).