Physics · Ch 6 — Optics
Polarisation by Scattering
Polarisation by Scattering
The light from a clear blue portion of the sky shows a rise and fall of intensity when viewed through a rotating polaroid, because sunlight scattered by air molecules changes its polarisation state depending on the scattering direction. The electric field of the incident (unpolarised) sunlight drives the electrons in an air molecule to oscillate in the plane perpendicular to the sunlight's own direction of travel; an observer looking at to the direction of the original sunlight can only receive radiation whose oscillation has a component transverse to the observer's own line of sight, since an accelerating charge radiates no energy along its own axis of oscillation -- so the light reaching this particular observer is …
What this figure shows. Unpolarised sunlight travels horizontally and strikes an air molecule; the molecule's electrons are driven to oscillate by the incident wave's electric field, in the plane perpendicular to the sunlight's own direction of travel. An observer positioned at 90 degrees to the original sunlight direction can only receive scattered light from oscillation components perpendicular to the line joining the molecule to the observer -- since an oscillating charge radiates no energy along its own axis of oscillation -- so the light this particular observer sees is shown as completely plane polarised, even though the origina …