Polarisation

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Advanced Higher Unit 3
Polarisation
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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.
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Unpolarised Waves For unpolarised waves the
oscillations are in many directions, e.g.
unpolarised light.
Unpolarised waves
Polarised wave
Note Only the electric field strength vector
only.
Effect of polarisation 1
Effect of polarisation 2
LCD displays
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We can check that the light is perfectly
polarised by using a second sheet of polaroid
called an analyser.
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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.
reflection
refraction
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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 Brewsters Angle.
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analyser
ip
ip
Later Brewster discovered that the polarising
angle that the reflected and refracted rays are
separated by 900.
air
Glass (refractive index, n)
r
normal
But r90-ip
gt
gt
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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.
ntan ip iptan-1(1.47) 560
r 90 ip 90 56 340
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