A desk lamp emits unpolarised light of irradiance

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

I_0 = 8.0\ \mathrm{W\,m^{-2}}

. A student places one ideal polaroid sheet in the beam and measures the transmitted irradiance, then adds a second ideal polaroid sheet with its transmission axis at

90^\circ

to the first (a crossed pair) and measures again. The student reasons: "Unpolarised light already contains every direction of vibration, so the first sheet cannot remove anything: it passes all

8.0\ \mathrm{W\,m^{-2}}

. Any polariser passes half of whatever irradiance reaches it, so the second, crossed sheet passes half of that,

4.0\ \mathrm{W\,m^{-2}}

." ((a)) Explain what is wrong with each step of this reasoning. ((b)) State the correct irradiance after the first sheet and after the crossed pair.

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