Stress patterns in a CD case revealed in polarised light. The bands of colour show mechanical stresses in the plastic that have been fixed in place during its setting from its liquid state. This occurs as the plastic displays a property called birefringence, in which light rays with different polarisations experience a different amount of refraction. The difference in the refraction is directly related to the stress in the plastic. The rays produced interfere with each other, causing some wavelengths to be amplified and some to be cancelled out, producing bands of different colours when viewed through a polariser. This technique, known as photoelasticity, is used to visualise the stress patterns in materials.