Topic 12:
P1.5.1 (continued)
The Electromagnetic Spectrum
The electromagnetic spectrum
The electromagnetic spectrum is a family of waves that travel without a medium!
They have many properties in common:
• They all travel at the speed of light
8
(3x10 m/s or 300,000km/s)
• They can travel through anything, and through vacuum easiest of all
(nothing to stop them)
• They are all waves, so they can be reflected, refracted and diffracted
Scary version!
You really shouldn't
learn any numbers!
th
1 nm = 1/1,000,000 of a millimetre
High energy
High frequency
Short wavelength
Visible light
mm
a r
Ga
Xr
ay
s
et
Bl
ue
Ind
igo
Vi
ole
t
n
ee
Gr
w
llo
ge
an
Or
Ye
are
Inf
r
ow
icr
d
M
Re
Vi
s
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ht
Ul
tra
vio
l
d
es
av
es
av
iow
Ra
d
Ah!
Much less scary!
ay
s
Low energy
Low frequency
Long wavelength
You need to learn
this sequence and
learn which end is
high energy/frequency
Radio Waves
• These are long wavelength, low energy waves.
• They are used in communications.
Radio waves are made when an alternating current flows in a conductor. As the electrons oscillate backwards and forwards they produce electromagnetic ripples called radio waves:
electrons
shake
radio
wave
electrons
shake
radio transmitter
radio receiver
electrical signal
electrical signal
• Shortwave requires expensve communications satellites
• Shortwaves do not
!
• Longwaves diffract around hills Win!
• Longwaves are poorer quality
• Shortwaves are higher quality
in
shortwave radio
• Longwave bounces n!
off the i
W
IONOSPHERE and land/sea
W
longwave radio
Microwaves
wavelengh = cms
ys
Radio waves
wavelengh <1m
­ might want to skip this page!
a r
a
Ga
mm
s
Xr
ay
Vi
s
lig ible
ht
Ul
tra
vio
Inf
rar
ed
s
M
icr
ow
av
e
es
wa
v
dio
• terrestrial TV
let
• national radio
• international radio
Great Radio waves
Radio waves
range wavelenghs >100m
wavelenghs 1­100m
Poor
quality
Ra
• satellite phones
• satellite TV
• live international TV
• local radio stations
• emergency services
(their range is limited)
Totally excessive detail.
Great quality
Poor
range
Communicating with light
...especially fibre optics.
Light reflects from side to side of the fibre
Compared to copper wires, fibre optics are:
• much cheaper ­ copper is expensive
• can be made thinner ­ more channels in the same space
• light signals in fibres don't interfere with each other like electrical signals do
• each fibre can carry more separate signals than a copper wire