Photoelasticity of Glass

Glass is the oldest man-made material. Its invention about five thousand years ago should be considered as one of the crucial events in the history of mankind. Glass has given man the possibility to have daylight in his protected living environment and to compensate the defects of his sight. Glass containers and tableware have played and still play an important role in man's everyday life. Glass elements in microscopes and telescopes have given us the possibility to learn the secrets of micro- and macrocosm. Glass participates in the most sophisticated technologies: glass fibers have caused a revolution in telecommunication, glass is used as a material for many modern electronic devices. Although nowadays plastics often make a strong competition to glass, for many applications glass is still the best material due to its specific properties - its hardness, good transparency, resistance to chemicals, the easiness to shape glass articles, feasibility to change the composition of the glass in order to meet new specific demands, etc. Two peculiarities of glass should be pointed out. The first is the fragility of glass - it breaks easily due to tensile stresses. The second is the fact that in every glass item there exist residual stresses due to the complicated technological process during which glass from the state of a viscous liquid at high temperature turns into solid state, while cooled down.