3.3 Electromagnetic spectrum
Gamma rays
Gamma rays (Topic 5.2) are more penetrating
and dangerous than X-rays. They are used to both
diagnose and treat cancer and also to kill harmful
bacteria in food and on surgical instruments. They
can also be used in engineering applications to
detect flaws in metals.
Dangers of electromagnetic
radiation
High exposures to electromagnetic radiation can
cause harmful effects.
wireless internet. Radio waves are weakened when
passing through walls, but are suitable for shortrange applications such as Bluetooth.
High-speed broadband internet and cable
television signals are carried by optical fibres
(Topic 3.2.2) because glass is transparent to
visible light and some infrared. Visible light and
shortwave infrared have higher frequencies than
radio waves so can transmit data at a higher rate.
Information is sent in digital form as a set
of pulses, rather than in analogue form as a
smoothly varying signal.
Microwaves
Analogue and digital
signals
Microwaves produce heat when they are absorbed by
the water in living cells. This can damage or kill the
cells. There is some debate at present as to whether
their use in mobile phones is harmful; ‘hands-free’
mode, where separate earphones are used, may be
safer.
There are two main types of signals – analogue
and digital.
In analogue signals, voltages (and currents)
can have any value within a certain range
over which they can be varied smoothly and
continuously, as shown in Figure 3.3.6a.
Infrared
a
+
Infrared radiation of high intensity can cause burns
to the skin.
voltage
Ultraviolet
Ultraviolet radiation absorbed in high doses can
cause skin cancer and eye damage. Dark skin is able
to absorb more UV in the surface layers than skin of a
lighter colour, so reducing the amount reaching deeper
tissues. Exposure to the harmful UV rays present
in sunlight can be reduced by wearing protective
clothing such as a hat or by using sunscreen lotion.
X-rays and gamma rays cause mutation or damage to
cells in the body which can lead to cancer. X-ray and
gamma ray machines are shielded by lead to reduce
unnecessary exposure.
Communication systems
Electromagnetic waves are the basis of many
communication systems. Microwaves only need
short aerials for transmission and reception of
signals, and can penetrate the walls of most
buildings, so are used for mobile phones and
time
–
b
+
voltage
X-rays and gamma rays
0
0
‘high’
‘low’
time
–
▲ Figure 3.3.6
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Analogue and digital signals
In digital signals, voltages have only one of two
values, either ‘high’ (e.g. 5 V) or ‘low’ (e.g. near
0 V), as shown in Figure 3.3.6b.
The advantages of using digital rather than
analogue signals to transmit information are
that they can be transmitted at a faster rate,
can be sent over longer distances since they can
be intermittently regenerated more accurately,
and are less affected by noise. Analogue signals,
such as sound waves, can be converted to digital
signals electronically before transmission.
Revision checklist
After studying Topic 3.3 you should know and
understand:
✓ that all electromagnetic waves travel with the
same high speed in a vacuum
✓ that the speed of electromagnetic waves is
3 × 108 m/s in a vacuum and approximately the
same in air
✓ the use of artificial satellites in communication
systems
Test yourself
3 Name one type of electromagnetic radiation which
a causes sun tan
b is used for satellite communication
c is used to sterilise surgical instruments
d is used in a TV remote control
e is used to cook food
f is used to detect a break in a bone.
4 Name one type of electromagnetic radiation which
causes
a damage to surface cells in the body
b internal heating of body cells
c burns to the skin
d mutation or damage to cells in the body.
5 Name one type of electromagnetic radiation
used for
a wireless internet
b Bluetooth
c cable television
d mobile phones.
✓ the use of specific types of electromagnetic
waves for different communication
applications.
After studying Topic 3.3 you should be able to:
✓ describe the different types of electromagnetic
radiation and distinguish between them in terms
of their wavelengths, properties and uses
✓ describe the harmful effects of different types of
electromagnetic radiation and how exposure to
them can be reduced
✓ distinguish between analogue and digital
signals and recall the benefits of digital
signalling.
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