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# The electromagnetic spectrum

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Light waves are a type of electromagnetic wave. They are part of a continuous electromagnetic (EM) spectrum that includes radio, microwave, infrared, visible, ultraviolet, X-ray and gamma ray radiations, as shown in the diagram below:

All electromagnetic waves are transverse waves which travel at the same speed through free space such as air or a vacuum. However, each of the wave types vary in frequency and wavelength and, as a result, each have unique and specific uses and potential detrimental effects.

As can be seen from the diagram, the EM spectrum has the radiation types arranged in order of their wavelength. As the speed for all of these wave types is equal at 3 x 108 m/s, their wavelength is inversely proportional to their frequency. At one end of the spectrum we have gamma rays with the lowest wavelength and therefore highest frequency. At the other end of the spectrum we find radio waves with the longest wavelength and therefore lowest frequency.

The order of the EM spectrum in terms of decreasing wavelength and increasing frequency can be summarised as shown in Table 1:

All of these EM radiations transmit energy. Their varied frequencies and wavelengths make them suitable for a wide variety of purposes. Excessive exposure to each type of EM radiation also has potential detrimental effects varying in severity. The higher the frequency of the radiation, the more harmful it is.

The uses of each type of EM radiation, along with the potential detrimental effects of excessive exposure, will now be discussed in detail starting with the least harmful and ending in the most harmful.

Radio waves have the longest wavelengths in the EM spectrum, ranging from 1 to 1 x 10-2 m. This type of wave is used mainly for broadcasting and communications. Radio waves are given out by a transmitter and are detected and received by aerials in devices such as TVs, and AM and FM radios. As radio waves have the longest wavelengths, they also have the lowest frequencies and are the least harmful type of EM radiation.

Excessive exposure to radio waves over a prolonged period of time is thought by some people to have the potential to cause cancer. However, there is currently much debate about this and no scientific evidence to support the belief has yet been found.

### Microwaves

Microwaves have the second longest wavelength in the EM spectrum ranging from 1 x 10-2–10-3 m, and are often used for cooking food and satellite transmissions. When food is placed in a microwave oven, the water molecules in the food absorb the microwaves and become very hot. The heat energy is passed between all of the particles allowing all of the food to become hotter. Microwaves can be used to carry signals between satellites and communication devices such as mobile phones.

Microwaves can cause internal heating of body tissues so excessive exposure can cause damage to tissues. There are some fears that excessive use of mobiles may cause heating of brain tissues leading to the development of brain cancer but there is currently no scientific evidence to support this. To protect against the risk caused by microwave radiation you can make sure that the microwave is turned off when not in use and use your mobile phone for a minimal amount of time when making calls.

### Infrared (IR)

Infrared radiation has the third longest wavelength in the spectrum at 1 x 10-3–10-6m.
Infrared radiation is transmitted by all objects as heat energy, including the human body. The hotter an object is, the more infrared radiation it will transmit. Infrared radiation is used to provide heat energy for heating homes or cooking food.

Infrared radiation can be used in night vision equipment. This type of equipment is often used from police helicopters when they are tracking criminals or searching for lost people. It can also be used to find survivors in disaster zones, as their bodies emit infrared radiation and can be detected on camera. Infrared radiation can also be used in remote controls for televisions, videos and other systems.

Excessive exposure to infrared radiation can cause skin burns. To minimise the risk of burns from heating elements, you can wear protective clothing such as oven gloves.

### Visible light

The wavelengths for visible light waves range from 1 x 10-6 –10-7m. All waves in the visible light region of the EM spectrum can be seen with the naked eye.

Visible light is used for many purposes but primarily it allows us to see other objects. It is particularly useful in photography and optical fibres such as those used in endoscopes used to see inside the human body. Visible light is made up of seven colours, each of which has a different wavelength. These colours are red, orange, yellow, green, blue, indigo and violet. Red light has the longest wavelength and the lowest frequency. Violet has the shortest wavelength and therefore the highest frequency.

Excessive exposure to intense visible light can damage the retina in your eye. For example, staring at a bright white light for too long may cause blindness. To prevent such damage occurring you should avoid looking directly into sources of intense bright light.

### Ultraviolet light (UV)

Ultraviolet radiation has a wavelength of around 1 x     10-7m. UV radiation is used on sunbeds and in fluorescent lamps.

As UV has such a short wavelength, its frequency is high. Most of the light emitted by the Sun is UV. UV has a high penetrating power which means that it can cause damage to the surface of the skin, and excessive exposure of skin to the Sun without protection can cause serious harm such as severe burns or skin cancer.

This is dangerous for the eyes as too much exposure can lead to blindness. The ozone layer is a protective layer of gases formed around the Earth which protects us from the most damaging types of UV radiation produced by the Sun. However, pollution has caused this layer to break down. Although the ozone layer can repair itself, this does not happen quickly and in some areas this breakdown is so severe that there are holes through which the harmful UV radiation can get to the Earth. These holes are bigger over some areas of the world and it is in these areas that there have been increases in the incidence of skin cancers.

To minimise the risks of exposure to UV you should always wear sun cream and if using a sunbed, minimise the time spent on the bed and avoid going too regularly.

### X-rays

X-rays have a very short wavelength of around 1 x 10-9m and therefore have a very high frequency. X-rays are very penetrating and pass through soft tissue easily, but do not pass through bones. X-rays can therefore be used to observe the internal structure of objects and materials. A common use of X-rays is by radiographers to check for broken bones in the bodies of human patients.

X-rays can also be used by airport security to check the insides of suitcases. Excessive exposure to X-rays can lead to cancer. To minimise the risks, hospitals will try and minimise the number of X-rays taken for patients and the radiographers who work with X-rays daily have to wear protective clothing and stand behind a lead screen to avoid overexposure. They also wear radiation exposure badges which monitor the amount of radiation they are being exposed to.

### Gamma rays

Gamma rays have the shortest wavelength of all of the EM radiations at approximately 1 x 10-13m.  Gamma rays have the highest frequency and are therefore the most penetrating waves.

Gamma rays can be used to sterilise food and medical equipment and to treat cancer in radiotherapy. As gamma rays are so penetrating they can cause damage to tissues below the surface of the skin. Gamma rays penetrate cells and cause their DNA to mutate which in turn can cause cancer.

Protective measures used when dealing with gamma radiation include use of thick lead shielding on protective clothing or screens.

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