A torch shines unpolarised light of irradiance through a polaroid card

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Polarisation and anisotropic media · Polarisation and anisotropic media

Optics and Photonics, Ch Polarisation states, polarisers and the Jones and Stokes calculus

Problem

A torch shines unpolarised light of irradiance 12 mW cm−212\ \mathrm{mW\,cm^{-2}} through a polaroid card. A second card is then added, rotated 90∘90^\circ relative to the first, to make a crossed pair, and the beam is measured again. A student argues: "The torch light is already unpolarised, so it has no particular direction for the first card to block; the first card must let it all through, 12 mW cm−212\ \mathrm{mW\,cm^{-2}}. Since a polariser always halves whatever hits it, the crossed second card then gives 6 mW cm−26\ \mathrm{mW\,cm^{-2}}." ((a)) Identify the error in each of the two claimed transmitted irradiances. ((b)) Give the correct transmitted irradiance after each card.

Browse the problem library