4. polarisation
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1) Plane polarized waves have their electric field strength vibrating in only one direction, as is the case with television waves transmitted in Britain. 2) Unpolarized waves have oscillations in many directions, as seen with unpolarized light. Polaroid sheets can be used to check if light is polarized by blocking the transmission of light if the polarization directions do not match. 3) Light reflected from insulating surfaces like glass becomes partially or fully polarized, with the angle of incidence and refractive index determining the amount of polarization. At Brewster's angle, no light passes through a second polarizing filter, showing the reflected light is plane polarized.
4. polarisation
- 1. Advanced Higher Unit 3
- 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 Useful web site