Optics > Polarization > Birefringence DCS# 6H35.50

STRESS BIREFRINGENCE

protractor between crossed polarizers

APPARATUS
two polarizers 202-21-E6
overhead projector 201
plastic protractor or ruler 101-34-D2
plexiglass rod or cylinder 202-18-D6
strained glass in plastic mount 202-21-E6
Polaroid photoelastic specimens kit 202-21-E6
piece of glad wrap 202-02-G2

DESCRIPTION
Show various clear plastic shapes between crossed polarizers on the overhead projector.  When squeezed or bent the plastic becomes anisotropic and therefore birefringent and bands of color can be seen.  The protractor shows colored bands without being deformed because anisotropies arise when the plastic solidifies when manufactured.  The colored patterns reveal internal stresses.

Place saran wrap between crossed polarizers and stretch along a direction 45° to the polarization axes to show that it becomes birefringent and allows a component of the light to pass.


NOTES
 
We have a commercial device to do the squeezing and stretching.