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the stress optic law is in the repo — it just needs a device to color the light.
strained glass
a loaded plate between crossed polarizers — the stress you cannot see, colored by the light it delays
polymer model material
a dark field: the plate is black until the load colors it
flat white, D65-like baseline
a clear plastic ruler bends and stays clear. put it between crossed polarizers and it fills with color. the load does not paint it — it delays one polarization against the other, and the second filter turns that delay into a color.
remove the analyzer and the plate is ordinary glass again. the black band is where the stress is zero; the bands crowd at the hole because the stress there is three times the load.
the polarizer rotates the crossed pair and sweeps the isoclinic black bands; the analyzer offset uncrosses it, opening the dark field to a bright one. the overlays show the field itself — the fringe order, the principal direction, the stress difference.