Polarisation by strain double refraction (birefringence)

Principle

During the manufacturing process, by subsequent treatment or the exertion of external forces it is possible to induce tensions in transparent materials, which bring about optical anisotropy of those materials. This means that a Iight wave train is split up into two parts with different oscillation planes when it passes through these materials. The propagation velocity of the Iight and hence the diffraction index are dependent on the direction of travel. This phenomenon is known as stress birefringence or photoelasticity.

In this experiment the students should realize that when polarized white Iight passes through optically anisotropic materials, coloured bright areas appear behind the crossed analyzer, and these are apparently brought about because the oscillation planes of parts of the emergent light are altered.

Benefits

• Multifunctional light box - All-in-one: Can be used for geometric optics on the table, colour mixing and on an optical bench
• Extension with others sets at anytime, no additional light sources needed, recognition value for students

Tasks

1. Direct polarized Iight through given photoelastic models, exerting on these at the same time various pressures which bring about inner tensions, and analyze the emergent light.
2. Then investigate whether spectacle lenses display inner tensions.