Double Refraction in Calcite, 6H35.15

Topic and Concept:

• Birefringence, 6H35. Birefringence

Location:

• Cabinet: Optics (OP)

• Bay: (A2)

• Shelf: #1

Abstract:

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°.

Discussion:

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

Videos:

• Lecture Demonstration's Youtube Channel

References:

• Hyperphysics http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/biref.html

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