Calcite Crystal - Iceland Spar
If you look at the words on a printed page through this crystal, you see double. These natural, nearly transparent crystals exhibit the property of 'birefringence', i.e., they can break light into two distinct polarized beams. By rotating a polarizing filter over the crystal, it is possible to view one image at a time! This is a great hands-on demonstration and wonderful when displayed using an overhead projector. Each crystal is about 2 cm x 2 cm x 4 cm.