C.K.Cheung
Polarization of Light
Wave nature of light:
Interference
diffraction
Particle nature of light: Photoelectric Effect
Polarization  Light waves are transverse in character.
Expt.:
step 1/
Use 1 polaroid: no effect when the polaroid is rotated about an axis  to
itself.
step 2/
Use 2 polaroids: one kept fixed and one rotated slowly.
fixed
Result:
Light is cut off in one position (  polaroids are crossed ), then rotate further
through 900  max. light transmission
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C.K.Cheung
Explanation:
unpolarized
plane polarized
If the right slit is further rotated 900:
unpolarized
plane polarized
plane polarized
Note:
A longitudinal wave would emerge from both slots whatever their relative
position. Hence, longitudinal wave cannot be polarized.
E.M. wave:
E
.
B
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C.K.Cheung
Experiment shows that the interaction of light with matter is more often through the
E.
 if light is vertically polarized  the plane containing E and the direction of travel,
called plane of vibration, is vertical.
In general:
plane of polarization ~ 109 times s- 1
E
resolve into 2  components:


unpolarize
Partially plane
polarize
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C.K.Cheung
Producing polarized light:
1/ By Polaroid
polarized light
unpolarized incident light
polaroid
Polaroid transmits light vibration, i.e. E vibration, in one particular plane and absorb
those in a mutually  plane.
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C.K.Cheung
2/
By reflection
when unpolarized light falls on glass, water and some other materials, the
reflected light is, in general, partially plane polarized.
But at one particular angle, called the Brewster’s angle, the polarization is
completed.
plane polarized reflected light
unpolarized
incident light

partially plane polarized
refracted ray
Formula:
tan  = n
For water:  = 530
For glass:  = 570
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C.K.Cheung
3/
By scattering
polarized (blue)
unpolarized
(yellow + red)
unpolarized
polarized (blue)
Equal amounts of polarized light are scattered in all sideways (  ) directions.
Note:
+
max.
intensity
_
F=qE
smaller
intensity
E=0
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C.K.Cheung
Using Polarized Light
1/
Polaroid disc, suitably oriented, are used in sunglasses and in photograph as
‘filters’ where they are placed in front of the camera lens, thereby enabling
details to be seen that would otherwise be hidden by glare
a/ no filter:
b/
with filter:
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C.K.Cheung
Blue sky!
EARTH
Scattering Effect 
1
4
 blue sky
Question: Why the sky is orange in colour at evening?
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C.K.Cheung
2/
To measure the concentration of optically active liquids ( e.g. sugar solution ) by
means of a polarimeter.

solution
  concentration
Measure the concentration in terms of the emergent intensity through the
polaroid.
3/
Stress analysis:
When certain substances (e.g. glass, perspex, polythene etc. ) under stress, they
become doubly refracting.
E (not obey Snell’s law)
O (obey Snell’s law)
Polaroid
plastic sheet
F
polaroid
F
If viewed in white light ( unpolarized ) between 2 crossed polaroids, coloured
fringes are seen round the regions of stress. The effect is called photoelasticity.
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C.K.Cheung
Classification of radio waves
long wave
f/Hz 30 k– 300 k
/m
103-104
Communication:
medium wave short wave
300 k –3 M
100 - 500
U.H.F.
microwave
3 M – 30M 30M – 300M 300M –
3000M
> 3000 M
10 - 100
too steep
an angle
gradual
refraction
n
f < 30 M Hz
V.H.F.
1-10
10 cm – 1 < 10 cm
f > 30 M Hz
Ionosphere
(80km-500km)
until completely
attenuated
transmitting
aerial
bounces
back
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C.K.Cheung
f: 30 k Hz ~ 30 M Hz ( below V.H.F.)
Long range communication ( ~ 106 m ) between countries.
f > fc ( critical frequency ) = 30 M Hz ( above V.H.F. )
No refraction, emitted waves go skywards.
Usually, fc depends on temp., pressure etc.,  fc varies with time of day,
sessions etc.. communication depends on environment ( disadvantage ).
Hence:
1/
for short range broadcasting, use:
a/
V.H.F. (  : 1m – 10m)
Local F.M.
b/
U.H.F. ( : 10CM – 1m)
TV ( carry picture and sound )
Range: ~ 105 m (if no obstacles)
  small, < 10 m  little diffraction from buildings, antenna
must points to station, especially for TV.
2/
For long range, use:
microwave, which can penetrate through the ionosphere, they are used in
radars and in communications between satellites and ground stations.
Advantage of satellite communications:
1/
2/
more dependable (  not affected by environment)
they contain amplifiers to boost the microwave signal used.
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C.K.Cheung
Antenna

4
Oscillator

4

4

4
reflector

4
Directors
Tuning circuit
Note:
1/
Long  waves usually show polarization because they are produced artificially
by generating currents in certain direction.
2/
Short  waves emitted by individual atoms  have a random mixture of planes
of polarization.
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C.K.Cheung
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