Here are some neat examples of a physical phenomenon known as birefringence. Many materials exhibit birefringence. However, these pictures are both of a carbonate mineral known as calcite.
Materials that exhibit birefringence are anisotropic. This means that they bend light at different angles depending on how the incident light is polarized. Since most light that we come into contact with on a daily basis is a combination of different polarizations, we see multiple images when we look through an anisotropic material like calcite.
More information on optics and birefringence:
http://en.wikipedia.org/wiki/Birefringence
http://www.physics.gatech.edu/frog (see the optics lectures)
http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html
Tags: birefringence, calcite, optics
