Polaroid filter 7x7 cm

Polaroid sheets are used to test whether there is tension in glass objects.

1. Place a polaroid on a light source, the polaroid sends the light rays all in 1 direction.
2. Above this, place/hold a 2nd polaroid.
3. Turn the polaroid until you no longer see any light.
4. Now hold the glass you want to test between the polaroids.

The polaroid sheets are sold per 1 piece (you will need 2).