When a transparent plastic sheet under mechanical stress is placed between two crossed Polaroids and illuminated with white light, coloured fringes are observed. Which property of the stressed plastic chiefly causes this effect?
(a) Stress-induced optical activity (rotation of plane of polarisation)
(b) Stress-induced double refraction (birefringence)
(c) Variable-thickness thin-film interference inside the plastic
(d) Surface diffraction from microscopic scratches on the sheet