Electromagnetic Spectrum


Radio Waves

Radio waves are a type of electromagnetic (EM) radiation with wavelengths in the electromagnetic spectrum longer than infrared light. They have have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, radio waves travel at the speed of light.

Naturally occurring radio waves are made by lightning or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radiocommunication, broadcasting, radar and other navigation systems, communications satellites, computer networks and innumerable other applications.

Large doses of radio waves are believed to cause cancer, leukaemia and other disorders. 

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Microwaves are basically extremely high frequency radio waves, and are made by various types of transmitter. In a mobile phone, they're made by a transmitter chip and an antenna, in a microwave oven they're made by a "magnetron". Their wavelength is usually a couple of centimetres. Stars also give off microwaves.

Microwaves cause water and fat molecules to vibrate, which makes the substances hot, so we can use microwaves to cook many types of food. Mobile phones use microwaves, as they can be generated by a small antenna, which means that the phone doesn't need to be very big. Wifi also uses microwaves. The drawback is that, being small, mobiles phones can't put out much power, and they also need a line of sight to the transmitter. This means that mobile phone companies need to have many transmitter towers if they're going to attract customers. Microwaves are also used by fixed traffic speed cameras, and for radar, which is used by aircraft, ships and weather forecasters. The most common type of radar works by sending out bursts of microwaves, detecting the "echoes" coming back from the objects they hit, and using the time it takes for the echoes to come back to work out how far away the object is.

Prolonged exposure to significant levels of microwaves is known to cause "cataracts" in your eyes, which is a clouding of the lens preventing you from seeing clearly, it not at all.

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Infrared Radiation

Infrared waves are just below visible red light in the electromagnetic spectrum ("Infra" means "below"). You probably think of Infra-red waves as heat, because they're given off by hot objects, and you can feel them as warmth on your skin. Infrared waves are also given off by starslampsflamesand anything else that's warm - including you. The detector on this security light picks up the Infra red radiation from your body.

They are used for many tasks, for example, remote controls for TVs and video recorders, and physiotherapists use heat lamps to help heal sports injuries. IR is also used for short-range communications, for example between mobile phones, or for the Dolby Screentalk headset system used in some cinemas. "Passive Infra-Red" (PIR) detectors are used in burglar alarm systems, and to control the security lighting that many people have fitted outside their houses.

The danger to people from too much Infra-Red radiation is very simple - overheating.

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Visible Light

Our eyes can detect only a tiny part of the electromagnetic spectrum, called visible light. This means that there's a great deal happening around us that we're simply not aware of, unless we have instruments to detect it. Light waves are given off by anything that's hot enough to glow. This is how light bulbs work - an electric current heats the lamp filament to around 3,000 degrees, and it glows white-hot. The surface of the Sun is around 5,600 degrees, and it gives off a great deal of light. White light is actually made up of a whole range of colours, mixed together. We can see this if we pass white light through a glass prism - the violet light is bent ("refracted") more than the red, because it has a shorter wavelength and is slowed down more by the glass - and we see a rainbow of colours. This is called 'dispersion', and allows us to work out what stars are made of by looking at the mixture of wavelengths in the light.

We use light to see things!

Too much light can damage the retina in your eye.

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Ultra- Violet

Ultra-Violet light is made by special lamps, for example, on sun beds. It is given off by the Sun in large quantities. We call it "UV" for short.

Uses for UV light include getting a sun tan, detecting forged bank notes in shops, and hardening some types of dental filling. When you mark your possessions with a security marker pen, the ink is invisible unless you shine a UV lamp at it. Ultraviolet rays can be used to kill microbes. Hospitals use UV lamps to sterilise surgical equipment and the air in operating theatres. Food and drug companies also use UV lamps to sterilise their products. Suitable doses of Ultraviolet rays cause the body to produce vitamin D, and this is used by doctors to treat vitamin D deficiency and some skin disorders.

Large doses of UV can damage the retina in your eyes, so it's important to check that your sunglasses will block UV light. Large doses of UV cause sunburn and even skin cancer. Fortunately, the ozone layer in the Earth's atmosphere screens us from most of the UV given off by the Sun. Think of a sun tan as a radiation burn!

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X- Rays

X-rays are very high frequency waves, and carry a lot of energy. They will pass through most substances, and this makes them useful in medicine and industry to see inside things. X-rays are given off by stars, and strongly by some types of nebula. An X-ray machine works by firing a beam of electrons at a "target". If we fire the electrons with enough energy, X-rays will be produced.

X-rays are used by doctors to see inside people. The machines are managed by a trained x-ray technician. They pass easily through soft tissues, but not so easily through bones.  X-Rays are also used in airport security checks, to see inside your luggage. They are also used by astronomers - many objects in the universe emit X-rays, which we can detect using suitable radio telescopes.

X-Rays can cause cell damage and cancers.

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Gamma Rays

Gamma rays are given off by stars, and by some radioactive substances. They are extremely high frequency waves, and carry a large amount of energy. They pass through most materials, and are quite difficult to stop - you need lead or concrete in order to block them out.

Because Gamma rays can kill living cells, they are used to kill cancer cells without having to resort to difficult surgery.

Doctors can put slightly radioactive substances into a patient's body, and then scan the patient to detect the gamma rays and build up a picture of what's going on inside the patient. Gamma rays kill microbes, and are used to sterilise food so that it will keep fresh for longer. This is known as "irradiated" food. Gamma rays are also used to sterilise medical equipment

Gamma rays cause cell damage and can cause a variety of cancers. They cause mutations in growing tissues, so unborn babies are especially vulnerable.

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The Electromagnetic Spectrum

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