Topic 5

What are the uses and hazards of waves that form the Electromagnetic Spectrum?

Features of Electromagnetic Waves

Any wave transfers energy from a point where there is a disturbance.  Drop a stone into water, and you will see the waves moving away from where the stone fell in (the disturbance).

Electromagnetic waves are like water waves.  They transfer energy from a source as waves.  They have an electrical component and a magnetic component, but you don't need to know the details of them at this stage.

All electromagnetic waves travel at the speed of light.

Speed of light (Physics code c) = 300 000 000 m/s = 3 × 10⁸ m/s

(Many very big or very small numbers in physics are written in standard form or scientific notation.  Make sure you know what this means.  Ask your physics or maths teacher if you don't.  Make sure you know how to enter standard form on your calculator.)

Electromagnetic waves travel in straight lines.

Unlike other types of wave, electromagnetic materials do not need a material to travel through.  They travel in a vacuum, which is why we see light from the Sun, but don't hear its roar.

All waves have a frequency.  This means the number of waves per second.  Frequency is measured in Hertz (Hz).

All waves have a wavelength.  This is the length of one complete wave, the distance between two successive peaks.  It is measured in metres (m).

The Electromagnetic Spectrum

Light forms a small part of a large family of electromagnetic waves.  You will know how light splits into the colours of the rainbow.  The scientific term for this is a spectrum.

You can see that the colours run into each other.  There are no distinct boundaries.

The rest of the electromagnetic spectrum is like this as well.  Here is a picture that sums up the electromagnetic spectrum (em-spectrum).

Notice these features:

• The boundaries on the diagram are not distinct.  Waves of wavelength 0.01 nm may be called X-rays or gamma-rays.

• The shortest wavelengths are on the left and the longest wavelengths are on the right.

• The most energetic waves are on the left, while the least energetic are on the right.

• Therefore the shorter the wavelength, the more energetic the wave is.

Uses of Electromagnetic Waves

Note the following conversions:

1 cm (centimetre) = 1 × 10^-2 m

1 mm (millimetre) = 1 × 10^-3 m

1 mm (micrometre) = 1 × 10^-6 m

1 nm (nanometre) = 1 × 10^-9 m

The boundaries between various radio frequencies are agreed internationally.  This is to stop stations from interfering with each other.  For example FM radio broadcasts occupy the frequency band 88 to 108 MHz.  Above 108 MHz the band is occupied by the aviation industry for communication and navigation aids for aeroplanes.

Note these conversions:

1 kHz (kilohertz) = 1000 Hz

1 MHz (megahertz) = 1 × 10⁶ Hz

Now answer QUESTION 1

How Electromagnetic Waves Behave

When an electromagnetic wave (radiation) hits a material, it can be:

• reflected, like light in a mirror;

• absorbed, like heat absorbed by a black surface;

• transmitted, or passed on, like light passing through glass.

Sometimes two, or even all three processes can occur.  Substances behave differently depending on what kind of radiation is falling on them, as the following examples show.

When energy is absorbed by a surface, it heats up.  For example microwaves are absorbed by water molecules and warm up.  This is how a microwave oven works.  Light waves simply pass through water.

A dark painted metal surface absorbs radio and light waves.  However X-rays and gamma rays can pass straight through.

Wax can transmit microwaves, but absorbs light waves.

Hazards of Electromagnetic Waves

We have seen that the shorter the wavelength, the more energetic the wave.  This can produce hazards, and we must take precautions to prevent these hazards from causing us harm.

Now answer Question 2.

Broadcasting  and Communication

The lower energy waves up to light can be used for telecommunications and broadcasts.  The wavebands for broadcasts are agreed internationally.  Other frequencies outside these are used for other purposes like telecommunications.

Question 3  How is light used for communication?  ANSWER

Microwaves are used for communications, for example telephones and computer links.

Infra red waves are absorbed by air, but are readily transmitted by glass.  Visible light is rapidly absorbed by glass.  Therefore infra-red is used for telecommunication by optical fibres. 

Optical fibres are very flexible and allow the infra red signals to travel around corners.

Analogue and Digital Signals

Communications and radio broadcasts use generally analogue signals.  This means that the signal can vary to any level as shown in the picture below:

This picture shows the complicated pattern made by speech. 

Digital signals have two levels, ON or OFF, 1 or 0.  Digital signals look like this:

Digital signals are used by computers and increasingly in communication and broadcasting.  They have a number of advantages:

• They are less prone to interference (where a radio signal distorts the sound and makes it unpleasant to listen to);

• They can be input directly to computers;

• The signal strength does not matter so much.

In other words digital signals are clearer.

We speak and hear in analogue signals, so we need converter circuits to convert analogue to digital and back again.

Question 4  What is the difference between analogue and digital signals?  ANSWER

The Wave Equation

There are three measured quantities in electromagnetic waves:

• The speed;

• The wavelength;

• The frequency.

They are linked by the following simple equation:

In triangle form:

The strange looking symbol that looks like an upside-down "y" is lambda, a Greek letter "l".  It is the physics code for wavelength.  The other codes are:

• c - wave speed (for electromagnetic waves c = 3 × 10⁸ m/s)

• f - frequency.

Worked Example

Question 5 What is the wavelength of a station that broadcasts on 95 MHz (95 000 000 Hz)?  ANSWER

Now try the Topic Quiz

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