The Electromagnetic Spectrum
Electromagnetic radiations are electric and magnetic disturbances. They travel as waves and move energy from place to place.
All electromagnetic waves travel through space (a vacuum) at the same speed but they all have different wavelengths and frequancy:
- Gamma rays have the shortest wavelength and highest frequancy.
- Radio waves have the longest wavelength and lowest frequancy.
- Different wave lengths of electromagnetic radiation are reflected, absorbed or trasmitted differently by different substances.
The electromagnetic spectrum in order of increasing wavelength:
Gamma rays, X-rays, ultraviolet, visible, infra-red, microwaves, radio waves.
All electromagnetic waves travel at 300 million metre per second.
Gamma Rays and X Rays
Gamma rays are used to sterilise surgical equipment and keep food fresh for longer by killing the bacteria on it.
X rays are used to produce show picture of bones, called radiographs.
Gamma rays and X rays mostly pass through soft body tissue, but some is absorbed and will damage the cells. In lower doses, both raditations can cause changes in cells that make them cancerous. In higher doses, they can kill the cells. Gamma radiation is used in hospitals, under carefully monitored conditions to kill cancer cells.
Working with these radiation is hazardous. Gamma sources are kept in thick lead containers. Staff should wear lead aprons and stand behind lead screens when using X Rays. They moniter the their exposure to the radiation with film badges.
Light and Ultraviolet Radiation
Ultraviolet radiation has a longer wavelength than X-rays. It has a shorter wavelength than the light at the violet end of the visable light spectrum.
Ultraviolet radiation from the Sun causes damage to the skin cells - tanning, sunburn, skin ageing and skin cancer. Over-exposure can also damage the eyes. Sun beds work by giving out UV rays.
Fluorescent tubes are coated with substances that absorb the ultraviolet radiation produced inside the tube. Then they emit the energy as visable light.
The same substances are used to make hidden sercurity marks that can only be seen by ultraviolet light.
Visable light is part of the electromagnetic spectrum that is deteted by our eyes. We see the different wave lengths within it as different colours; Visible light can be transmitted along optical fibres.
Infra-Red, Microwaves and Radio Waves.
- Infra-red Radiation is given out by all objects. The hotter the objects, the more infra red it emitts. Night vision equipment works by detecting this radiation. Infra red is absorbed by the skin, we sense it as heat and it can burn. It is used as the heat source in toasters, grills and radiation heaters. TV, video and other remote controls use Infra red, It can be transmitted along optical fibres.
- Mircowaves are used for cooking and in commuications. Microwaves ovens produce frequancies that are absorbed by water molecules. They heat the water in food, cooking it from inside out. The water in living cells will absorb microwaves and they may be damaged or killed by the heat released. Microwave transmitters produce wave lengths that are able to pass through the atmosphere. They are used to send signals to and from satelilites and within mobile networks.
- Radio waves are used to transmitt radio and TV programmes. Whenn an alternating voltage is appiled to an aerial, it emitts radio waves with the same frequancy as the alternating voltage. When the waves are received they produce an alternating current with the same frequancy as the radiation.
The microwave and radio wave part of the electromagnetic spectru is used for communications. The includes terrestrial TV, satellite TV, mobile phones, emergancy services radio, amateur radio transmitters, local, national and international radio.
Different frequancies are used for different applocations.
Optical fibres are very thin glass fibres. They are flexible and can be bent around curves. Light or infra red radiation is transmitted alon the fibre by continous reflextions.
Analogue and Digital Signals
Communications signals are either analogue or digital.
- An analogue signal varies continously in amplitude.
- Digital signals only have certain values, usually thet are either high (on or 1) or low (off or 0). They can be processed by computers.
When signals are tranmitted they become less strong over distance and have noise amplifed. They also pick up noise. When amplification takes place the noise is also amplifed. With analogue this can make the signal very distorted. With digital the noise can be 'cleaned' because it is clear which part of the signal is high and which is low.
Digital signals are shorted than analogue so more information can be passed ditigally as many 'flashes' can be passed every second.