Detailed Notes
Parts of the electromagnetic spectrum including radio waves, visible light and infra-red are used in communication. Radio waves easily pass through clouds and fog and can be sent long distances through the atmosphere from the radio transmitter to the receiver. Interference occurs if two radio stations transmit on the same frequency, leading to regulation of the parts of the electromagnetic spectrum by authorities. One of the other main problems to be overcome with radio waves is the curvature of the earth.
Microwaves are used for mobile phone communication. They cannot be broadcast like radio waves, so the receiver needs to be in the line of sight of the transmitter. The transmitter and receiver need to be very high (e.g. on top of the BT tower) to reduce buildings, hills or the earth’s curvature blocking the rays. In the UK there are 200 microwave stations each about 50 km apart to account for the earths curvature.
Short wave radio waves are used to transmit TV and FM radio. The signal is sent from the transmitter on the earth’s surface to geostationary satellite which is placed at the same position above the earth. The satellite reflects the signal to the receiver on a different part of the earth’s surface. Short wave radio can only be sent if the satellite is in ‘view’ of both the transmitter and receiver, so it cannot be sent around the earth in one go. The signal is however high quality and sharp. This type of radio wave will not diffract over hills or far through buildings.
Medium wave radio signals bounce off part of the earth’s atmosphere called the ionosphere. They can travel long distances around the earth, but the signal is often poor due to interference.
Long wave radio waves diffract (bend) along the earths surface, allowing transmission across long distances. They also diffract over mountains and into buildings and tunnels.
Infra-red is also part of the electromagnetic spectrum, however as it has a very short wavelength compared with radio waves so it is easily absorbed and scattered by the atmosphere.
Many undeveloped countries make use of satellite communication to connect the many isolated villages, as it is cheaper than laying thousands of miles of copper or fibre cable. Even developed countries usually only have fibre optic cable as the backbone to their networks, with copper delivering the signal to the individual houses.
Development of Transatlantic Telephone Calls
Communication between England and America used to be very difficult, and was generally by post or telegraph. The advent of transatlantic telephone calls has made communication far easier.
History of transatlantic telephone calls (England and America):-
1940s – short wave radio was bounced off the layer of the atmosphere (ionosphere) – the operator spent up to an hour to connect a 3 minute call. The line quality was poor and ‘crackled’ due to interference.
1950s – the first transatlantic copper cable was laid. It allowed 36 conversations at a time. The calls were still poor quality due to interference. Many repeater stations were needed to boost the signal.
1960s – present day – Transatlantic calls are made by satellite. A transmitter sends microwaves carrying encoded voice signals from 1 country, to a satellite in space which reflects the signal to the receiver in the receiving country. The main problem with satellite communication is the time delay of about half a second.
1970s – Europe’s first optic fibre telephone link came into service, it was laid by British Telecom.
1980s – First transatlantic optical fibre entered use which quickly transfers high quality sound.
1980s – First optical repeaters came into use, which boosts the light signal without having to convert it back to electricity and then light. This greatly improves the speed of transmission.
1990s – First transpacific optical fibre entered use which quickly transfers high quality sound.
The Colour Spectrum and Filters
White light is made up of many colours which have slightly different wavelengths (by combining the three primary colours - red, green and blue, you get white light).
Our eyes detect the different wavelengths and our brain decodes this as a certain colour.
Wavelength is measured in nanometres (nm).
White light can be separated into its different colours using prisms and/or filters. Coloured filters let through some wavelengths of light but stop others depending on the colour of the filter. For example, a green filter will only let through green light. A yellow filter will let both green and red light through as yellow light is made up of both these colours.
Filters are very useful for making a receiver sensitive to one colour of light only. This means many messages can be sent at the same time - a process known as multiplexing.
