Sketch the pattern of isochromatic regions. Which of the isochromatic regions corresponds to the unstressed region of the bar?
To determine the birefringence use a Michel-Levy chart, whose axes are retardation (birefringence times thickness) versus the thickness of the specimen. On the chart are plotted lines of constant birefringence. The thickness of the bar was 3.0 mm. Use the Michel-Levy chart to determine the optical path difference at the midpoints of the two long edges of the bar and deduce the optical path differences arising from the stress at these points. Hence estimate the stress at these points using the Stress-Optic Law
\[n_Q - n_P = C(\sigma_P -\sigma_Q) \]
The principal stresses, σP and σQ, in the plane section are aligned parallel to orthogonal directions P and Q; nP is the refractive index for the vibration direction parallel to P and nQ is the refractive index for the vibration direction parallel to Q.
The stress-optical coefficient of polycarbonate is -78.0 × 10-12 Pa-1.