Sample of Iceland spar, early 19th century.
© Science Museum / Science & Society Picture Library
A rhomboidal crystal of Iceland spar, a mineral that is also known as calcite. It strongly exhibits double refraction, a phenomenon also known as birefringence. This can be seen as the double image of the circle and cros on the test card lying underneath the transparent crystal. Due to its nature, light is refracted or bent along two different crystal planes within the calcite mineral. First noticed in the 17th century, its causes were misunderstood until the French physicist, Augustin Fresnel (1788-1827) explained its occurrence using the wave theory of light.