Electromagnetic waves - reflection, refraction, diffraction
- a summary or tutorial about the basics of the way in which electromagnetic waves are reflected, refracted and diffracted
As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.
Reflection of light is an everyday occurrence. Mirrors are commonplace and can be seen in houses and many other places. Shop windows also provide another illustration for this phenomenon, as do many other areas. Radio waves are similarly reflected by many surfaces.
When reflection occurs, it can be seen that the angle of incidence is equal to the angle of reflection for a conducting surface as would be expected for light. When a signal is reflected there is normally some loss of the signal, either through absorption, or as a result of some of the signal passing into the medium.
A variety of surfaces can reflect radio signals. For long distance communications, the sea provides one of the best reflecting surfaces. Other wet areas provide good reflection of radio signals. Desert areas are poor reflectors and other types of land fall in between these two extremes. In general, though, wet areas provide better reflectors.
For relatively short range communications, many buildings, especially those with metallic surfaces provide excellent reflectors of radio energy. There are also many other metallic structures such as warehouses that give excellent reflecting surfaces. As a result of this signals travelling to and from cellular phones often travel via a variety of paths. Similar effects are noticed for Wi-Fi and other short range wireless communications. An office environment contains many surfaces that reflect radio signals very effectively.
It is also possible for radio waves to be refracted. The concept of light waves being refracted is very familiar, especially as it can be easily demonstrated by placing a part of stick or pole in water and leaving the remaining section in air. It is possible to see the apparent change or bend as the stick enters the water. Mirages also demonstrate refraction and a very similar effect can be noticed on hot days when a shimmering effect can be seen when looking along a straight road. Radio waves are affected in the same way. It is found that the direction of an electromagnetic wave changes as it moves from an area of one refractive index to another. The angle of incidence and the angle of refraction are linked by Snell's Law that states:
For radio signals there are comparatively few instances where the signals move abruptly from a region with one refractive index, to a region with another. It is far more common for there to be comparatively gradual change. This causes the direction of the signal to bend rather than undergo an immediate change in direction.
Radio signals may also undergo diffraction. It is found that when signals encounter an obstacle they tend to travel around them. This can mean that a signal may be received from a transmitter even though it may be "shaded" by a large object between them. This is particularly noticeable on some long wave broadcast transmissions. For example the BBC long wave transmitter on 198 kHz is audible in the Scottish glens where other transmissions could not be heard. As a result the long wave transmissions can be heard in many more places than transmissions on VHF FM.
To understand how this happens it is necessary to look at 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. It is found that diffraction is more pronounced when the obstacle becomes sharper and more like a "knife edge". For a radio signal a mountain ridge may provide a sufficiently sharp edge. A more rounded hill will not produce such a marked effect. It is also found that low frequency signals diffract more markedly than higher frequency ones. It is for this reason that signals on the long wave band are able to provide coverage even in hilly or mountainous terrain where signals at VHF and higher would not.
By Ian Poole
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