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The Electromagnetic Spectrum

Light is one member of a family of waves called the electromagnetic spectrum.

All types of electromagnetic radiation can travel across a vacuum and travel at the same speed through space, 300.000.000 m/s or 300,000 km/s.

The different types of radiations have different wavelengths. This affects how they are reflected, absorbed or transmitted by different substances and types of surface.
 

When radiation is absorbed, the energy it carries causes the substance to get hotter.

It may also create an alternating current with the same frequency as the radiation itself.

These, and other properties, determine what each type of radiation can be used for and what its effects are.

Type of Radiation

Uses

Wavelength

Frequency

Gamma rays

  • To kill harmful bacteria in food.
  • To sterilise surgical instruments.
  • To kill cancer cells.

Shortest Wavelength

Highest Frequency

X-rays

  • To produce shadow pictures of materials that X-rays do not easily pass through, e.g. bones and metals.

 

 

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Ultra-violet rays

  • In sunbeds, because u-v causes skin to darken.
  • Special coatings which absorb u-v and emit light are used in fluorescent lamps and security coding.

Visible light

  • Lighting systems.
  • Optical fibres, e.g. endoscopes, so doctors can see inside patients’ bodies.

Infra-red rays

  • In grills, toasters and radiant heaters.
  • In optical fibre communication.
  • For remote control of TVs and VCRs.

Microwaves

  • Those with wavelengths strongly absorbed by water are used in microwave ovens.
  • Those that can pass easily through the Earth’s atmosphere are used to send information to and from satellites and in mobile phone networks.

Radio waves

  • To transmit radio and TV programmes between different points on Earth.
  • Longer wavelength waves can be reflected off an electrically charged layer in the upper atmosphere, to be sent between distant points, despite the curvature of the Earth.

Longest Wavelength

Lowest Frequency

Optical fibres can carry more information than sending electrical signals through cables of the same diameter. There is also less weakening of the signal.

Effects on Living Cells

Microwaves are absorbed by the water in cells. The heat released may damage or kill the cells.

Infra-red radiation is absorbed by the skin and felt as heat. If a lot is absorbed it can cause burns.

Ultraviolet radiation causes tanning. It can pass through skin to deeper tissues where it can cause cell damage which may develop into skin cancers. High doses kill normal cells.

Darker skins absorb more ultraviolet. Less reaches the deeper tissues, so there is less risk of cancer.

X-radiation and gamma radiation mostly passes through the soft tissues, but some is absorbed by the cells. High doses kill cells. Lower doses of these types of ionising radiation can cause normal cells to become cancerous.

Digital and Analogue Signals

Speech or music can be converted into electrical signals to be transmitted long distances through cables or using electromagnetic waves (radio waves) as carriers.

Information can also be converted into light or infra-red signals and sent along optical fibres.

As signals travel they become weaker and additions to the signal (noise) may be picked up.
 

Signals which vary continuously in amplitude and /or frequency are called analogue signals, e.g. the sound waves of speech and music.

The different frequencies within the signal may weaken by different amounts. Each time the signal is amplified these differences, and any noise picked up, are also amplified.

In other words, the signal becomes less like the original. Its quality deteriorates.

Signals can be coded into a series of pulses. The signal can only be on or off. These are called digital signals.
 

Digital signals have a higher quality than analogue signals. The information they carry is not changed during transmission.

Although the pulses weaken with distance, they are still recognisable as ‘on’ or ‘off’.

Any noise picked up is generally of low amplitude, so is interpreted as off and is ignored.
 

More information can also be sent in a given time as a digital signal, whether it is by cable, optical fibre or carrier wave.

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