This is called double refraction or birefringence . When you look at something through a birefringent substance, you can see a double image. Iceland spar, a type of calcite crystal, is birefringent. Below, you can see a double image as you look through a piece of Iceland spar. Click on the crystal to move it over the pen and text.
Scientists use many tools to look at things like crystals.
One of those tools is a microscope. There are many things we can learn by looking at crystals in different ways. There are ways of looking at birefringent crystals through a microscope that can create very colorful and beautiful images. Visit our Using Microscopes to Investigate Birefringence in Crystals activity site to learn how you can view these images. To learn more about refraction of light, visit our Refraction of Light Interactive Java Tutorial.