Doubly refractive materials

Other interference-produced colors falling into this section include doubly refracting materials such as anisotropic crystals and strained isotropic media between polarizers, as in photoelastic stress analysis and in the petrological microscope. 

FIG. 35 The phase behavior of dodecane sulfonic acid with water. O, Doubly refracting material appears as the isotropic solution is cooled. <3, Isotropic solution appears as the liquid crystal is heated. CD, Crystalline solid disappears on heating. Pairs of solid points connected with the vertical dashed lines mark the appearance and disappearance of pseudoisotropy as samples are heated. The area ABC, within which the intermediate mesophase exists alone, is not precisely determined. 

The distribution of stresses can be clearly revealed in photo-elastic models of joint assemblies made from a transparent material that becomes doubly refractive when subjected to stress. If a beam of polarised white light is shone through a stressed component, coloured lines appear when the object is viewed through a second polarisation filter as a result of interference effects. These lines indicate areas of equal stress. If stress levels are increased, the sequence of coloured lines is repeated. The photographs below show specimen components made from clear acrylic or polycarbonate joined by different methods. In the side view of a bolted coimecfion under load, the stress peaks around the bolt appear very clearly (Fig. 13). 

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