PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Polarisation and anisotropic media · Polarisation and anisotropic media
Optics and Photonics, Ch Birefringence and crystal optics
Problem
On an optics bench, a half-wave plate (Γ=π) is mounted with its fast axis at 15∘ to the horizontal. Horizontal linearly polarised light (polarised at 0∘) is incident on it.
((a)) Without multiplying out the Jones matrices, use the reflection rule for a half-wave plate to state the direction of the output polarisation, measured from the horizontal.
((b)) A photoelastic model of a cantilever bracket, of thickness d=6.0 mm (the thickness the light travels through), is loaded so that the stress-induced index difference n1−n2 grows with distance x from the free end as Δn(x)=Δnmax(x/L)2, reaching Δnmax=2.0×10−4 at the clamped end (x=L). The model is viewed in light of wavelength λ=589 nm between crossed polarisers set at 45∘ to the bracket's principal stress directions, so that isoclinic bands are absent. State, with a reason, whether the isochromatic (dark) bands are more closely spaced near the free end or near the clamped end, and find the total number of dark isochromatic bands (orders m=0,1,2,…) seen along the bracket.