Polariscope

Feeling stressed?

Like a wave on a string, light is a transverse wave---the direction the wave "wiggles" is always perpendicular to the direction it moves (Figure a). Because of this, a group of light waves can be polarized, meaning they all wiggle in the same direction as they move.

Much as a narrow window only lets a string "wiggling" in a certain direction pass (Figure b), materials called polarizers only let light with a certain polarization direction pass. Here, you're seeing two perpendicular polarizers (the monitor and the screen), which ideally don’t let any light through. This is why the monitor appears dark through the screen.

Materials like glass and plastic can rotate light’s polarization direction (Figure c), letting more pass through the screen. The patterns arise because the color and amount of light let through depends on the stresses present in the material. This is called stress birefringence.

Stress birefringence is used to study how materials react to bending and stretching to find potential points of failure. It can also be used to identify minerals and gemstones in a process known as polarimetry.