Photoelasticity, also known in some circles as mechanical birefringence is a phenomenon in which normally transparent isotropic substances can be made optically anisotropic by the application of mechanical stress. It was discovered in 1816 by Sir David Brewster. First, we look at transparent isotropic substance. This means, roughly, that if you have a material and you shine light on it it almost doesn't matter where the light hits the material, your outcome is the same. A mirror reflects an object whether the object is up high or down low. A telescope magnifies an image whether it's straight out from the lens, or off at an angle. In other words you have many optical axes(Plural of axis?). Now, by placing mechanical stress on an object, you can change the isotropic properties of a material. Specifically you confine the optical axis to one direction. Under compression you see results similar to a negative uniaxial crystal; tension gives results similar to a positive uniaxial crystal. The sign aspect indicates where light would be reflected or refracted to in relation to the material. Positive indicates in front of the material, negative indicates behind. This is largely arbitrary though, and is used only in complex problems with many materials, since you could choose any side of the material to be the front. So what does all this mean. It means that materials under stress cannot acurately focus or reflect light incident upon them. The severity of this birefringence is proportional to the stress and the matrial used. Car windshields and telescope lenses routinely have highly photoelastic substances bonded to them to easily detect even the smallest amounts of birefringence that could be caused by the strain of improper mounting.