Get Your Science On: Crystal Clear

This week, Fey shows us a neat experiment called Crystal Clear. Using a special transparent crystal, you get an double image. However, when you put on polarized glasses, things get cleared up!

What’s Going On:

We are shining unpolarized light on the calcite crystal, and this leads to a phenomenon called double refraction. Light is a wave, and as it moves through space, it can wiggle in any direction. For simplicity we can say the unpolarized light is wiggling both horizontally and vertically. We can see two images, one for each of the polarizations of light coming in. Wearing the polarized glasses eliminates one of the overlapping images and clears up the picture!

A Bit More Technical:

As light goes from air to a material, it effectively slows down due to interactions with the atoms that make up the material.  The amount light gets slowed down is related to a property of the material called the index of refraction.The calcite crystal has a property called birefringence, which means that the light gets slowed down by different amounts depending on its polarization. We would say that the index of refraction of calcite crystal depends on the polarization of the light. Vertical and horizontally polarized light travel at different speeds, and this is what leads to the double image.


  • a polished (transparent) calcite crystal
  • polarized sunglasses or a linear polarizing filter
  • something to look at
  • a light (optional)

How To Build:

This one’s pretty straightforward to build. Look at something through the calcite crystal both without and with the polarizers.