1) Virtual height:- The incident and refracted rays follow paths that are exactly the same as they have been if reflection had taken place from a surface located at a greater height, called Virtual height of this layer
2) Actual height:- The actual height of the wave in the ionized layer is a curve and is due to refraction of wave. The height from this curve to earth surface is called actual height.
3) Critical frequency: The critical frequency of a layer is defined as the maximum frequency that is returned back to the earth by that layer, when the wave is incident at an angle 900 (normal) to it. The critical frequency for F2 layer is between 5 to 12 MHz.
4) Maximum usable frequency: The limiting frequency when the angle of incidence is other than the normal is known as maximum unstable frequency. MUF= fc secθ