Science

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.

What is a spectrum?

The visible spectrum

a rainbow appears to come out of the prism

Refraction from a prism

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.

The electromagnetic 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.

The discovery of infrared

Sir William Herschel. Credit: John Russell

National Maritime Museum London UK

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.

The discovery of ultraviolet

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.

The main types of electromagnetic radiation

X-rays, visible light and radio waves are all types of electromagnetic radiation.

The main types of electromagnetic radiation

frequencytype of electromagnetic radiationwavelength
highestgamma radiationshortest
X-rays
ultraviolet
visible light
infrared
microwaves
lowestradio waveslongest

All types of electromagnetic radiation:

  • are transverse waves
  • travel at the same speed in a vacuum - empty space.

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.

Hazards of electromagnetic radiation

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:

  • microwaves cause internal heating of body tissues
  • infrared radiation is felt as heat and causes skin burns
  • X-rays damage cells, causing mutations (which may lead to cancer) and cell death
  • gamma rays also damage cells, causing mutations (which may lead to cancer) and cell death.

Microwaves

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.

Watch

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 light

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.

The three main types of ultraviolet radiation, and some of their effects

typefrequencyhazard
UV Chighcauses severe damage to cells
UV Bmediumcauses severe sunburn and damage to cells
UV Alowweaker effects than UV B

Uses of electromagnetic radiation

Some uses of electromagnetic radiation

Electromagnetic radiationUses
radiowavesbroadcasting

communications

satellite transmissions

Satellite with solar panels in earth orbit
microwavescooking

communications

satellite transmissions

A microwave
infraredcooking

thermal imaging

short range communications

optical fibres

television remote controls

security systems

A remote control
visible lightvision

photography

illumination

A camera
ultravioletsecurity marking

fluorescent lamps

detecting forged bank notes

disinfecting water

Counterfeit money detection
X-raysobserving the internal structure of objects

airport security scanners

medical X-rays

X-ray of hands. Credit: Ron Chapple
gamma rayssterilising food and medical equipment

detection of cancer and its treatment

Woman receiving gamma camera scan

Exam tip

You may be asked about uses of electromagnetic radiation. To make your answer as full as possible you should include:

Ionising radiation

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:

  • detect smoke
  • gauge the thickness of paper
  • treat cancer
  • sterilise medical equipment.

Types of radiation

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:

  • alpha
  • beta
  • gamma.

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 cannot pass through humans, beta is stopped by aluminium, gamma by lead

Penetrative properties of different types of radiation

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

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.

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