When we insert a hard plastic item into the field between the laptop and the camera, the birefringent property of the plastic causes the light's angle...
By using a pair of polarizing filters and a property of certain materials called birefringence we can photograph the hidden stresses in hard plastics. This instructable was inspired by this article and a comment I read in another instructable, somewhere, about LCD monitors and polarization.
Step 1: What you will need
A laptop or LCD monitor. LCD's have a built in polarizing filter, and will act as our light source.
In a darkened room. Turn on the laptop and create a white screen, I used clipboard on a Thinkpad 380 running win98, or you can create a white image in an editor and view it full screen. Mount the camera on the tripod and point it at the laptop screen, zoom in until the screen fills the camera's field of vision. If you haven't already, fit the polirizing filter onto the camera, slowly rotate the camera filter until the laptop screen goes black (you need to use the lcd on the camera, an optical viewfinder won't turn black ;-)). When the screen appears black from the camera's perspective we've achieved "cross polarization". Now we can start with the pictures.
Step 3: Taking the pictures
When we insert a hard plastic item into the field between the laptop and the camera, the birefringent property of the plastic causes the light's angle of polarization to change, since our light is no longer "cross polarized" it becomes visible again to the camera. And since different wavelengths have different refractive indices the visible light appears as a rainbow of colors. Exposures are on the order of 1/2 second so a stand should be used to hold the object.