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The photograph shows a polycarbonate sample containing a sharp crack. The
sample is under tensile stress and is being viewed in polarized light.
The stress in the material makes the dielectric properties anisotropic
and this anisotropy causes the plane of light polarization to rotate, giving
rise to the fringes shown. The density of the fringes is a measure of the
local stress field, high fringe densities corresponding to high local stresses.
The colors arrise from the spectral content of the light and the wavelength
dependence of the refractive index of the material.
From:
"Materials Science and Metallurgy" Cambridge University
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