1.

Stress analysis.

Certain plastics rotate the plane of polarisation of light passing through them. The angle through which the plane of polarisation is rotated is found to vary when the sample is placed under stress, for example by bending it. The angle also depends on the wavelength of the light.

The first photograph below shows a small part of a plastic set square viewed under normal conditions.

The next photograph shows the same object when placed between crossed polaroids.

At places where the internal stresses are greatest, the coloured bands change more rapidly (with distance). Engineers wishing to predict where a mechanical component might fail, when placed under stress, can make a model of the component out of, for example, Perspex and observe the “stress patterns” (the coloured bands are more “concentrated” where the stresses are greatest. The design of the model can then be modified accordingly before the actual component is manufactured.

2.

Measuring the concentration of solutions

Certain solutions rotate the plane of polarisation of light passing through them. The angle through which the plane of polarisation is rotated depends on the concentration of the solution.

3.

Liquid crystal displays.

On the under-side of the top plate of the liquid crystal container, A, are fine lines etched parallel to the plane of polarisation of the top Polaroid. Similarly there are lines etched into face B parallel to the plane of the lower Polaroid.

The liquid crystals line up with these fine lines but also tend to line up with each other so there is a gradual change in the alignment of the crystals from A to B. The crystals change the plane of polarisation of the light so that the light can pass through the lower polariser and be reflected by the mirror. The display appears light.

When a voltage is applied, the crystals line up along the direction of the electric field and they therefore no longer allow light to pass through. The display appears dark.