Unpolarised white light of initial intensity $I_{0}$ passes through three ideal linear polarisers. The first polariser's transmission axis is vertical, the third's is horizontal (i.e. the two are crossed). The middle polariser is set at an angle $\theta$ to the vertical and is not ideal: it transmits only $90\%$ of the incident polarised light (the remaining 10 % is absorbed).

(a) Find the angle $\theta$ that maximises the transmitted intensity after the third polariser.
(b) Determine the maximum transmitted intensity as a fraction of $I_{0}$.

Question

Unpolarised white light of initial intensity $I_{0}$ passes through three ideal linear polarisers. The first polariser's transmission axis is vertical, the third's is horizontal (i.e. the two are crossed). The middle polariser is set at an angle $\theta$ to the vertical and is not ideal: it transmits only $90\%$ of the incident polarised light (the remaining 10 % is absorbed).

(a) Find the angle $\theta$ that maximises the transmitted intensity after the third polariser.
(b) Determine the maximum transmitted intensity as a fraction of $I_{0}$.

Accepted Answer

(a) $\theta = 45^{\circ}$. (b) Maximum transmitted intensity = $0.1125\,I_{0}$ (i.e. 11.25 % of the incident intensity).

Gce Model Questions Vol1 2019 Physics Question 48

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