= Solution
The local slow direction follows the radial rods. Between crossed <polarizers>, the angular factor is $\sin^2(2\varphi)$, so radial directions parallel to either polarizer are dark. The pattern is a \b[<Maltese cross in polarized-light microscopy>], with four bright sectors separated by dark arms parallel to the polarizer directions. Its colours depend on the local <optical path difference>; tapering rods or changing fill fraction can make this vary with radius. The cross belongs to the local radial-axis pattern and does not imply a <conoscopic interference figure>.
\Image[/past-exam-of-the-materials-science-course-of-the-university-of-cambridge/2019/paper-1-form-birefringence.png]
{title=Form birefringence, wedge compensation and a radial extinction cross}
{description=Original idealized optical model. The radial image shows normalized monochromatic intensity, not a simulated photograph or measured colour chart.}
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