Polarisation AH Physics QW Mr Stewart Electromagnetic Waves

Polarisation AH Physics Q&W Mr Stewart

Electromagnetic Waves �An electromagnetic wave is comprised of an electric field component and a magnetic field component. These components are perpendicular to each other Most light sources for example sunlight are unpolarised. This means the electric and magnetic field vectors are emitted in random planes. This is usually shown like this: When studying polarisation we only need to consider the electric field vector.

Polarisation �Sunlight (or any other unpolarised light) can be polarised by passing it through a polariser. This is a filter that has lots of very thin, closely packed parallel lines on it that only allows light through in one plane. After passing through the polariser this light is plane polarised

Polariser and analyser �A second filter (identical to the first) can be used as an analyser. �This means it can be rotated to allow the plane polarised light through or to block it. The transmission of polarised light depends on the orientation of the analyser. �When the transmission axis of the polariser and the analyser are perpendicular, no light is transmitted through the analyser.

Rotating the analyser �As the analyser is slowly rotated the transmitted light will have an intensity that follows a cos 2 curve. In other words: The polariser and analyser are initially oriented so that the maximum light is transmitted…… If the analyser is then rotated 90° the intensity falls to zero. If the analyser continues to be rotated it will be back to a maximum intensity at 180° and so on.

Applications of polarisation 3 D Movie glasses - watch Polaroid sunglasses reduce glare from reflected sunlight. This is because when sunlight hits the surface of the water it is plane polarised on reflection… The sunglasses act like an analyser to the polarised reflected light and block any reflected “glare” (Try shining light on a tray of water and viewing the surface through a polaroid sheet. )

Polarisation by reflection �Unpolarised light can be plane polarised by reflection from any flat transparent electrical insulator ie glass, water, perspex. �The degree of polarisation depends on the incident angle and the refractive index of the material. �For a specific angle of incidence the reflected light is 100% plane polarised. This incident angle is called the Brewster Angle (Scottish Scientist –Sir David Brewster 1781 -1868) Reflected light is 100% plane polarised when the angle between the reflected ray and the refracted ray is 90° (see next slide)

Derivation of Brewster angle equation From Snells Law – Higher Physics At the Brewster angle i =θB , the angle between the reflected ray and the refracted ray is 90° and since: the angle of incidence = the angle of reflection Where ip is the polarising or Brewster angle and n is the refractive index

Example Calculate the angle for incidence for light hitting the surface of water so that the reflected light is 100% plane polarised. (n=1. 33) (In other words calculate the Brewster angle for water) Solution: