In some transparent materials the application of a stress changes the refractive index for light travelling through that material. If the magnitude of the applied stress is different in different directions in the material the refractive index will also vary with orientation. This “Photoelastic Effect” has useful applications, for example in observing the variations in magnitude and orientation of the stresses in samples. Some examples of such observations have been presented and the underlying science required to understand them has been introduced. In particular the concept of “permitted vibration directions”, each having a different refractive index for light travelling through an anisotropic material has been presented and the resulting “phase difference” (or equivalently “optical path difference”) has been identified as providing the key to understanding the patterns of fringes observed in the stressed samples. A fuller discussion of the propagation of light through anisotropic materials is presented in the TLP “Introduction to Anisotropy”.