P1.5 The Electromagnetic Spectrum

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Radio Waves

Radio waves have the lowest frequency and the longest wavelength.

They are produced by various types of transmitter, and are also generated naturally by lightning, stars and other astronomical objects. This is why you will hear some interference on your radio during a thunderstorm.

Radio waves are mainly used for communication. Depending on their wavelength, radio waves can be divided into categories with different applications:

  • Medium wavelengths of around 100m are used for AM radio stations, such as BBC Radio 5.
  • Smaller wavelengths of around 2m are used by more common, FM radio stations. Most stereo radio stations use these, as well as taxis and some civilian aircraft.
  • Ultra High Frequency (UHF) radio waves have wavelengths of less than a metre. These are used by military aircraft radios, police radios, and television transmissions.

Large doses of radio waves can cause some cancers such as leukemia.

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Microwaves have frequencies of 0.3 - 300GHz and wavelengths of a couple of centimetres. They are artificially generated by transmitters, but are also produced by stars.

Their most common application is cooking. This is because microwaves cause water and fat molecules to vibrate when they pass through them, generating heat energy.

Microwaves are also used in mobile phones, speed cameras and radar systems used by aircraft, ships and weather forcasts. One type of radar works by releasing large amounts of microwaves at once and then detecting their "reflection" off the objects which they come into contact with, using their known wave speed to determine how far away objects are.

Dangers of microwaves include cataracts - a clouding of the lens of the eye resulting in blindness - which the prolonged exposure of this type of radiation has been known to cause. There has also been research which has shown that exposure to microwaves can cause brain tumours.

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Infra-red radiation is named so because it is just below the visible red light on the electromagnetic spectrum (infra meaning below). They have frequencies of 300GHz - 430 THz and wavelengths of upto 1mm.

Infra-red radiation is a type of heat energy transfer, and is therefore given off by hot objects such as lamps, stars, flames... even the warmth of your skin. Thermal imaging cameras used by the police and security systems are able to detect infra-red waves emitting from a criminal's body.

Other uses of infra-red cameras include weather forcasting, and finding people that have become trapped following a natural disaster.

The remote control that connects to your television also uses infra-red, and if you took a photograph of the little bulb in the end of the remote using a thermographic camera you would be able to see this as a small light.

The only real danger of too much infra-red radiation is overheating.

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Visible Light

This is the very tiny section of the electromagnetic spectrum which our eyes can detect. Its wavelength ranges from 400 - 700nm and its frequency is between 430 and 770THz.

Depending of the wavelength of the light wave the colour will appear different, with the shortest wavelengths corresponding to the colour violet, and red light having the longest wavelength. This rainbow of colours can be viewed by passing white light through a glass prism. This is because white light is a combination of all the colours, and when it passes through the prism the violet light is refracted most and the red light is refracted least, meaning that the colours separate and are clearly visible.

Visible light is extremely useful - it is how we see things around us. It is produced by very hot objects, such as lightbulbs and the Sun. Visible light rays can also be produced by lasers, such as in CD players and laser printers.

The danger of too much visible light is temporary or permanent blindness caused by damage to the retina. This is usually caused by looking directly at the Sun for too long.

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Ultra-Violet or UV rays are given off in large quantities by the Sun. They are also artificially produced by "fly catcher" lights and artificial tanning beds.

Its wavelength is from 10 - 400nm and its frequency ranges from 750THz to 30PHz.

Ultra-violet is useful for security reasons. This is because authorised bank notes have an image printed onto them in a chemical which is invisible to us but can be detected by shining UV rays through the paper, meaning that forged notes can be spotted for not having this image. If you have ever used a UV pen to mark valuable items in your house in the event of a burglary, you have used this same chemical.

UV rays also kill microbes, and are therefore used to sterilise medical equipment and foods. Another valuable property of this type of wave allows them to be used to harden some types of dental filling.

Large doses of UV can cause damage to the retina, severe sunburn and some skin cancers. This is why overuse of sun beds can be very dangerous to health.

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X Rays

X rays are high frequency waves that give off a lot of energy. They have the second shortest wavelength on the spectrum, ranging from 0.01 to 10nm. X rays are given off by stars and some types of nebulae.

They are used by doctors to see inside your body and find anomalies in the skeleton, such as broken bones. This is because x rays can pass through soft tissue but are absorbed by bone. The rays are passed through a person's body and onto a film which turns black when in contact with x rays. Wherever the rays are able to pass through the body the film will go dark, and where they are absorbed by the bones the film remains white. This creates an image of the skeleton.

In large amounts, x rays can cause cell damage and even cell mutation - a process leading to the formation of tumours and development of cancer. Because of this, several precautions must be in place in hospitals using x ray machines:

  • Radiographers must stand behind a lead screen or wear a lead apron to protect themselves.
  • Patients are limited to the number of x rays they can have per year.
  • Rooms containing x ray machines must have clear warning signs above the door.
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Gamma Rays

Gamma rays are given off by stars and by some types of radioactive decay. They have the highest frequency and the shortest wavelength, meaning they carry lots of energy. They can pass through most materials and are very difficult to stop - a large block of concrete or lead is needed.

Gamma rays can kill cells, and for this reason they are used to treat cancer. This is because healthy body cells are able to repair themselves quickly after the effects of radiation, but cancer cells can't. This is called radiotherapy, and the dose must be very highly controlled to reduce damage to healthy cells.

Gamma rays kill microbes, so are used to sterilise food to keep it fresher for longer. This is known as "gamma irradiation". They are also used to sterilise medical equipment.

Despite being a cure for cancer, in the wrong dosage gamma rays can actually cause cancer, by stimulating the mutation of cells and the formation of tumours. This is why radiotherapy must be heavily controlled by expert doctors.

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