The electromagnetic spectrum
White light can be split up into a spectrum of many different colours and visible light is just part of a continuous spectrum of electromagnetic radiation. Different types of electromagnetic radiation have different hazards and uses.
Using a prism, you can split up white light to form a spectrum. (A prism is a block of glass with a triangular cross-section.) The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.
Visible light is just one type of electromagnetic radiation: there are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through space.
British astronomer William Herschel (1738-1822) was making observations of the sun when he put coloured filters over his telescope in order to make his observations safer. He noticed that different coloured filters heated up his telescope by different amounts. Using a prism to break up visible light he put a thermometer in the different colours. He found that the temperature rose as he moved the thermometer from violet to red. He then measured the temperature where there was no visible light, at the red end of the spectrum. The temperature was the highest: he had discovered infrared.
Following Herschel’s work, Johann Ritter (1776-1810) tried to find invisible rays at the violet end of the spectrum. As part of the experiment he used silver chloride, which turns black when exposed to light. This happened fastest when exposed to the invisible rays at the violet end of the spectrum.
X-rays, visible light and radio waves are all types of electromagnetic radiation.
|frequency||type of electromagnetic radiation||wavelength|
All types of electromagnetic radiation:
The speed of electromagnetic radiation in a vacuum is 299,792,458 m/s. This is approximately three hundred million metres per second - nearly nine hundred thousand times faster than sound, which is why you see a flash of lightning before you hear the thunder.
Over-exposure to certain types of electromagnetic radiation can be harmful. The higher the frequency of the radiation, the more damage it is likely to cause to the body:
Microwave radiation can be used to transmit signals such as those for mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile phones in their range.
There is concern that microwave radiation from mobile phones and masts may be harmful to health.
You may wish to view this BBC news item (2006) about this issue.
Ultraviolet radiation - UV - is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens as our bodies attempt to reduce the amount of ultraviolet radiation reaching deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important, because ultraviolet radiation can cause normal cells to become cancerous.
|UV C||high||causes severe damage to cells|
|UV B||medium||causes severe sunburn and damage to cells|
|UV A||low||weaker effects than UV B|
short range communications
television remote controls
detecting forged bank notes
|X-rays||observing the internal structure of objects|
airport security scanners
|gamma rays||sterilising food and medical equipment|
detection of cancer and its treatment
You may be asked about uses of electromagnetic radiation. To make your answer as full as possible you should include:
Radioactive substances give out radiation all of the time. There are three types of nuclear radiation: alpha, beta and gamma. Alpha is the least penetrating, while gamma is the most penetrating. Nonetheless, all three are ionising radiation: they can knock electrons out of atoms and form charged particles.
Radiation can be harmful, but it can also be useful - the uses of radiation include to:
Nuclear radiation comes from the nucleus of an atom. Substances that give out radiation are said to be radioactive. All radiation transfers energy. There are three types of nuclear radiation:
Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, while gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out.
The thicker the substance, the more radiation is absorbed. The three types of radiation penetrate materials in different ways.
Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.
Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.
Check your understanding by having a go at this animation. Click on each image of the rock to discover the reading on the radiation meter. Use the readings to confirm that the rock gives out beta radiation.