A polariscope allows for the visualization of internal stresses in an object using the polarization of transverse electromagnetic (light) waves. Here it is comprised of an LCD panel used as a source of linearly polarized light and a camera covered by a linear polarizer sheet oriented to block the polarized light.
Because different wavelengths (color) and polarizations of light propagate through a medium at different speeds (birefringence), the polarization axis of the light is rotated differently depending on color as it passes through the object. The resulting changes in polarization caused by the object allow the camera to image certain colors of light.
Since the internal stress in the glass object causes local variations in its birefringence properties, the image seen by the camera corresponds to these stresses. If the glass object had been rotated completely around to properly image the crack, there would have been little color surrounding it.
Source: https://www.instagram.com/p/4bvG3oognr/
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