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 l and is measured in
metres.
The wave equation
wave speed (metre/second, m/s) =
frequency (hertz, Hz) × wavelength
(metre, m)
wavespeed = fl
• 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 = fl
• 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 l
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.