Question

The (usual) modes of propagation of EM waves, having a frequency. (i) less than a few MHz (ii) from a few MHz to (30–40) MHz (iii) above 40 MHz are respectively, 

(1) Ground waves, space waves, sky waves 

(2) Ground waves, sky waves, space waves

(3) Sky waves, space waves, ground waves 

(4) Space waves, ground waves, sky waves 

Answer 1

Correct option: (2) Ground waves, sky waves, space waves

Explanation: 

In the ground wave mode of propagation the wave glides over the surface of the earth. This mode can be used only for frequencies less than a few MHz. 

The sky wave uses the ionospheric reflection of radio waves; the ionosphere does not reflect back waves of frequencies more than (30–40) MHz. 

The space wave mode of propagation is used for line sight communication and satellite communication is done primarily through this mode only.