Table of Optics Demonstration

List of Optics Equipment & Supplies

Lecture Demonstrations

Double Refraction in Calcite, 6H35.15

Topic and Concept:



Rotate crystal of calcite (also known as Iceland Spar) above printed text. Two images are visible; one remains fixed as the crystal is rotated, the other circumnavigates the first.

In the above left picture, see the "S" in "Spar" forms two images, which lie on a diagonal line running from upper left to lower right.

In the above right picture, the two images of the "S" now lie on a diagonal line running from upper right to lower left.

The fixed image is due to the so-called "ordinary" ray. The mobile image is due to the so-called "extraordinary" ray. (It's not easy to tell in the above pictures which image is fixed and which is mobile).

If a linear polarizer is available, it can be used to show that each ray is polarized, and that the angles of polarization of the two rays differ by 90°.



ID Number

Calcite crystal

OP, Bay A2, Shelf #1

Linear Polarizer

OP, Bay B5, Shelf #2


The mathematical description of birefringence seems too advanced for an introductory course, however the demonstration may catch students' interest.