t of some of the signal passing into the medium. For radio signals surfaces such as the sea provide good reflecting surfaces, whereas desert areas are poor reflectors. Refraction of radio waves is obviously very similar to that of light. It occurs as the wave passes through areas where the refractive index changes. For light waves this can be demonstrated by placing one end of Radio waves and propagation 27 Direction of Direction of incident wave reflected wave Oj = Or Figure 2.8 Reflection of an electromagnetic wave a stick into some water. It appears that the section of stick entering the water is bent. This occurs because the direction of the light changes as it moves from an area of one refractive index to another. The same is true for radio waves. In fact the angle of incidence and the angle of refraction are linked by Snell's law that states: In many cases where radio waves are travelling through the atmosphere there is a gradual change in the refractive index of the medium. This causes a steady bending of the wave rather than an immediate change in direction. Diffraction is another phenomenon that affects radio waves and light waves alike. It is found that when signals encounter an obstacle they tend to travel around them as shown in Figure 2.10. The effect can be explained by Huygen's principle. This states that each point on a spherical wave front can be considered as a source of a secondary wave front. Even though there will be a shadow zone immediately behind the obstacle, the signal will diffract around the obstacle and start to fill the void, thereby enabling reception behind the obstacle even though it is not in the direct line of sight of the transmitter. It is found that diffraction is more pronounced when the obstacle approaches a 'knife edge'. A mountain ridge may provide a sufficiently sharp edge. A more rounded obstacle will not produce such a marked effect. It is also found that low frequency signals diffract more markedly than higher frequency ones. Thus signals on the long wave band are abl