Materials: Two polarizing filter, clear plastic fork or stress demonstrator, and wood.

Building Instructions: Place two pieces of wood close together to hold filter upright. Repeat for other filter. Place clear plastic fork or stress demonstrator between filters and squeeze them. Notice the colors produced for the pattern of stress created.

Functions and use: Light normally vibrates in all directions but sometimes it vibrates in only one direction – this is called polarized light. It turns out that when light reflects off the road or a car or other object it becomes polarized. That is why polarizing filters are used in sunglasses, it filters out the polarized light that is due to the reflection (this reflected polarized light we call glare). This concept is used a lot in radio astronomy and sometimes in other radiation types as well.

In this demonstration, a clear fork is placed between two pieces of polarizing filters that are set-up so that they are opposite of each other. If the fork (or stress demonstrator) is squeezed (carefully squeeze the fork’s tines – don’t break them though!), the squeezing changes the speed of the light as it travels through the fork and this small change allows some of the light to travel through the second filter and creates the colors seen. Any imperfection shows up when the fork is stressed in this way. This is often used in industry to test for stress points in objects before manufacturing begins.