Advanced Higher Unit 3

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.

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 2 Unpolarised wavesPolarised wave LCD displays

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

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 ipip air Glass (refractive index, n) normal reflection refraction

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, i p, or Brewster’s Angle. Brewster angle analyser ipip ipip air Glass (refractive index, n) normal

Later Brewster discovered that the polarising angle that the reflected and refracted rays are separated by But r=90-i p => analyser ipip ipip air Glass (refractive index, n) normal r

Example Glycerol has a refractive index of Find: (a) the polarising angle for glycerol (b) The angle of refraction at the polarising angle. n=tan ip ip=tan -1 (1.47) =56 0 r = 90 – ip = 90 – 56 = 34 0 Useful web site