A: Wave Phenomena

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A: Wave Phenomena. A.6 Polarisation. Polarisation When a charged particle loses energy, a tiny disturbance or ripple in the surrounding electromagnetic field is created – an e-m wave.
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A: Wave PhenomenaA.6 PolarisationPolarisationWhen a charged particle loses energy, a tiny disturbance or ripple in the surrounding electromagnetic field is created – an e-m wave. The wave will be in the same plane as the plane of movement of the particle. If all the disturbances in a ray of light were occurring in the same plane, the light is said to be plane polarised.In reality most sources of light produce unpolarised light because the electrons in the source move in all directions.electronElectromagnetic wavesPolaroidsA Polaroid filter (a polariser) polarises light rays. If an unpolarised ray of light passes through the filter, about 50% of its energy will be absorbed and 50% pass through. Planes before:Plane after:The original light waves exist in many different planes. The polarising filter only allows components of the wave in one plane to pass through.
  • E.g.
  • One wave in its original plane:
  • This has vertical and horizontal components:
  • - Wave after passing through filter with a vertical plane of polarisation (horizontal component absorbed):
  • Polarisation by ReflectionWhen an unpolarised ray of light is reflected off a non metallic surface, the reflected ray becomes partly polarised. E.g. Reflections of light from cars, the sea, snow etc.The degree of polarisation is dependant upon the angle of incidence of the light.Brewster’s AngleWhen light reflects off water, polarisation will be 100% when the angle of incidence is equal to Brewster’s angle (ϕ – phi). This occurs when the angle between the reflected and refracted rays is 90°.ϕϕrOn the right hand side of the normal:
  • ϕ + r = 90
  • but...
  • so...
  • but... sin (90 – ϕ) = cosϕ
  • so Brewster’s angle is given by...
  • n2= sin i sin rn = sin ϕ sin (90 – ϕ)n = sin ϕ = tan ϕcosϕϕ = tan-1 nE.g.Calculate Brewster’s angle for diamond (n = 2.4).ϕ= 67°Malus’ LawUnpolarisedlight of intensity I0 can be split into two parallel components which in turn may be polarised by a second polarising filter (called an analyser). The resulting intensity (I) is given by Malus’ Law:I = I0 cos2 θθ = angle of rotation between the two polarisers. I = intensity, the power per unit area (Wm-2)I0E.g. A sheet of Polaroid is used to reduce the intensity of a beam of polarised light. What angle should the transmission axis of the Polaroid make with the plane of polarisation of the beam in order to reduce the intensity of the beam by 50%?
  • so... I = ½ I0
  • and I = I0 cos2 θ
  •  ½ I0 = I0 cos2 θ
  • ½ = cos2 θ
  • cosθ = √(0.5)
  • θ = cos-1 √(0.5)
  • θ = 45°
  • Polaroid SunglassesYou were sunglasses to protect your eyes, but the benefit of wearing them is to block glare. Glare is light that is polarised by reflection. They absorb the horizontally plane polarised glare off the snow, water, or other surfaces that would normally make you squint. Stress AnalysisGlass and plastics become birefringent (double refraction) when placed under stress. When polarised white light is passed through stressed plastics and then an analyser, bright coloured lines are observed in the regions of maximum stress.Demo in classLiquid Crystal Displays (LCDs)LCDs use a series of reflectors, polarisers, electrode plates, and liquid crystals. The liquid crystals are optically active substances that rotate the plane of polarisation of light that passes through it.Examples:CalculatorsWatchesScreensTVs SubtitleTextIntensity I0Intensity ISubtitleTextSubtitleText
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