The Electromagnetic Spectrum

            Light is only one part of a group of waves called electromagnetic waves. They are similar because they all travel at the same speed in a vacuum, and are all transverse. The only difference is in the wavelength, which can vary from more than 1000m (radio) to less than 10^-12m (X and gamma rays). The shorter the wavelength (the higher the frequency), the more dangerous they are. There are six different types of waves: radio, infra-red, visible light, ultra-violet, X-ray and gamma waves.

Radio waves:

Radio waves have the longest wavelength, ranging from 1000m to 10^-3m. There are several kinds of radio waves.

Microwaves are the shortest of radio waves. Microwaves are used for cooking and in telecommunications as well as for radar. Then there are TV waves called UHF (Ultra High Frequency) waves and VHF (Very High Frequency) waves, used to transmit local radio programs. There are also long and medium radio waves used to transmit over long distances, due to their ability to diffract easily round hills and mountains. Some sources of radio waves are: radio transmitters, including radar and TV transmitters, and microwave ovens.

Infra-red rays:

            All objects emit infra-red radiation. The hotter an object is, the more energy it emits. Your own body emits infra-red radiation. In medicine, infra-red scanners are used to detect ‘hot spots’ under the body’s surface. They can often mean that the underlying tissue is unhealthy. Infra-red radiation is just beyond the red part of the visible spectrum. Its wavelength varies from 10^-3m to about 10^-6m.

Visible light:

            This is the only part of the spectrum which we can see. The wavelength is around 5 x 10^-7m. Its sources are hot objects, the Sun, florescent substances, lasers and LED’s.

Ultra-violet rays:

            Ultraviolet (or UV) radiation is harmful to our eyes. Solar radiation contains UV radiation as well as light. UV rays tan the skin, but does not heat it like infra-red radiation. However, too much UV radiation causes sunburn. It can also cause skin cancer.

Ozone in the atmosphere absorbs much of the ultraviolet radiation from the Sun, but there is much concern about the damage done to the ozone layer caused by pollution.

Ultraviolet radiation makes certain chemicals glow. Such chemicals in washing powder make clothes look brighter in sunlight. The disco lighting that makes white clothes glow also uses UV radiation. The wavelength of UV rays is usually around 10^-8.  

X-rays:

            X-rays are produced by an X-ray tube. In Medicine, X-radiation is used to take radiographs or X-ray pictures of limbs and organs. X-rays pass through tissue but they are stopped by bones. They can also be used to destroy a tumor inside the body. However, much X-radiation can induce cancer. The typical wavelength of an X-ray is of 10^-10m.  

Gamma rays:

            Gamma rays have the shortest wavelength: 10^-12m. They are very penetrating and extremely dangerous to human beings. They are produced by an radioactive substance such as uranium. If used with great care, they can sterilize food so that it does not rot so quickly as well as check that two pieces of metal are welded together properly. In Medicine, a carefully controlled beam of gamma-rays can be used to kill cancer cells.

back up