Coloured interference patterns in a plastic model of a prosthetic (artificial) hip joint are visible when photographed through polarised light. Photoelastic studies on models are used in the stress analysis of real components and structures. When viewed through polarised light certain plastics display interference fringe patterns from which the direction and magnitude of stresses may be calculated. These calculations may then be related to stresses in real components, whether they be manufactured in steel, aluminium, cast iron or many other materials.