Coloured light interference pattern of a knife and fork. When photographed through cross-polarised white light some transparent plastics display coloured interference effects, according to the pattern of residual stress within the plastic (a result of moulding manufacturing). Areas of similar colour represent regions under similar degrees of stress. This phenomenon, known as photoelasticity, is utilised in engineering situations. Plastic models of bridges, gears, axles, etc may be made and subjected to proportional loads in order to predict the stress patterns that might occur in the real objects.