Optics > Polarization > Birefringence DCS# 6H35.50
STRESS BIREFRINGENCE
protractor between crossed polarizers
APPARATUS
two polarizers 105-23-E6
overhead projector 105
plastic protractor or ruler 105-34-D2
plexiglass rod or cylinder 105A
strained glass in plastic mount 105-23-E6
Polaroid photoelastic specimens kit 105-23-E6
piece of glad wrap 105A

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.