The Electromagnetic Spectrum

Electromagnetic radiation is a disturbance in an electric field. This results in a magnetic field at right angles to the electric one. Therefore it is called an electromagnetic radiation – electric and magnetic disturbances result!

It propagates (moves) through space carrying energy from one place to another

All electromagnetic waves (also called electromagnetic photons) can travel through a vacuum – it is the only way that energy can propagate through space

The waves cover a continuous range of wavelengths called the electromagnetic spectrum. They are called different names according to how they are produced and have a wide range of wavelengths

Wavelength is the smallest distance between two corresponding points on a wave. It is given the symbol and is measured in metres.

The wave equation wave speed (metre/second, m/s) = frequency (hertz, Hz) × wavelength (metre, m) wavespeed = f

All electromagnetic radiation travels through a vacuum at the same speed – the speed of light – symbol ‘c’ – which is 300,000,000 m/s This is the fastest speed anything can travel at!

As electromagnetic waves all travel at the same speed and: c = f If a wave has a long wavelength it must have a low frequency and vice versa a high frequency wave must have a short wavelength… f and have to always equal 300,000,000 when they are multiplied together.

When electromagnetic rays travel through a medium (such as light travelling through glass) interaction with the particles in the glass delays the energy transfer and the speed is slower. This results in refraction. Higher energy photons are slowed down more than lower energy ones and therefore refract more.

The uses and hazards of the radiations in different parts of the electromagnetic spectrum depend on their energy. The higher energy they have the shorter the wavelength will be and the higher the frequency will be. High energy – high frequency – short wavelength

Radiation is either reflected, absorbed or transmitted when it hits an object. Different substances and types of surface have a different effect on various parts of the electromagnetic spectrum of radiation. E.g. – glass transmits most of the visible light that falls on it, reflects some and absorbs very little – but it transmits very little UV radiation – therefore you cannot get a suntan through a window!

When radiation is absorbed by an object, the energy it carries makes the object hotter – see the Sun’s energy melting ice in the picture. It may also create an alternating current with the same frequency as the radiation itself within that object – that is how an aerial works.

Different wavelengths of electromagnetic radiation have different effects on living cells. Some radiations mostly pass through soft tissue without being absorbed (gamma rays and X-rays). Some produce heat (microwaves and infra red). Some may cause cancerous changes (ionising radiation – gamma, X-rays and high energy UV) and some may kill cells (High doses of ionising radiation). These effects depend on the type of radiation and the size of the dose.

Radiowaves, microwaves, infra red and visible light can be used for communication.

Microwaves can pass through the Earth’s atmosphere and are used to send information to and from satellites and within mobile phone networks.

Infra red and visible light can be used to send signals along optical fibres and so travel in curved paths. Endoscopes use this to see inside patients without cutting them open (we will do this as a practical when I get back!)

You have to know the uses and dangers of all of the parts of the electromagnetic spectrum. Here is a table that lists these in an easy to learn format. If you look at this table on Cyberphysics you will find links to pages that give you more detail – but this is what you need to learn.

Microwave danger Whether biological effects other than just heating take place is debatable - some scientists think it may increase the risk of tumour growth - but if this is true this is not by the route recognized to do so (it is none ionizing) Some work has been done investigating the effect it has on the immune system and some scientists think that it has an effect on it by depressing that system and therefore allowing tumour growth to occur that would usually be dealt with easily by the body.

Microwave danger Effects such as short term memory loss have also been investigated. Scientists cannot say a link has been established until they can show a causal link – until cause and effect is understood by them! An independent government report recommended using mobile phones for the minimum time possible until we know whether it is a danger – especially for children as they are most likely to develop cancers if the fear is shown to be correct. They want us to put the precautionary principle into practice.

Precautionary principle The precautionary principle is a moral and political principle which states that if an action or policy might cause severe or irreversible harm to the public, in the absence of a scientific consensus that harm would not ensue, the burden of proof falls on those who would advocate taking the action.