2. An electromagnetic waves
The wave is plane polarised, or linearly polarised. The
electric field strength is in one direction only.
In Britain this is the way that T.V. waves are transmitted.
Aerials are designed and oriented to pick up the vertical
electric field strength vibrations.
Note, only transverse waves can be polarised, since
longitudinal waves oscillate in the direction in which they
travel.
3. Unpolarised Waves
For unpolarised waves the oscillations are in many
directions, e.g. unpolarised light.
Note: Only the electric field strength vector only.
Effect of polarisation 1 Effect of polarisation
Unpolarised waves Polarised wave
LCD
displays
4. We can check that the light is perfectly polarised by using
a second sheet of polaroid called an analyser.
Polariser
Unpolarised
light
Axis of
transmission
Polarised
light
Analyser
Axis of
transmission
eye
No light
transmitted
5. Polarisation by Reflection
Light reflected from the surface of an electrical insulator
is partially and sometimes fully polarised.
The angle of incidence and the refractive index decide the
amount of polarisation.
Consider a beam of unpolarised light reflected from a
sheet of smooth glass.
Brewster angle
analyser
ipip
air
Glass
(refractive index, n)
normal
reflection
refraction
6. At a certain angle of incidence no light passes through the
analyser showing that the reflected light is plane polarised.
This angle is called the polarising angle, ip, or Brewster’s
Angle.
Brewster
angle
analyser
ipip
air
Glass
(refractive index, n)
normal
7. Later Brewster
discovered that the
polarising angle that
the reflected and
refracted rays are
separated by 900
.
But r=90-ip
=>
=> pin tan=
analyser
ipip
air
Glass
(refractive index, n)
normal
p
p
i
i
n
cos
sin
=)90sin(
sin
p
p
i
i
n
−
=
r
i
n
p
sin
sin
=
r
8. Example
Glycerol has a refractive index of 1.47. Find:
(a) the polarising angle for glycerol
(b) The angle of refraction at the polarising angle.
n=tan ip
ip=tan-1
(1.47)
=560
r = 90 – ip
= 90 – 56
= 340
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