Tomsk State University. Physical Department.
Laboratory of training physics experiment
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Wood as a  Birefringence Crystal       

The crystal optics issues, conception of circular and elliptical polarization are difficult for students comprehension. In the laboratory of Tomsk state university the demonstration equipment kit had been created, where the wood was used as a birefringence crystal for the microwave (32 mm).

The direction of the layers in the wood determines the crystal optical axis. A segment of an ordinary pine board is similar to the optical plate cut out of a crystal parallel to the optical axis (number 2 on the photo). The end-circle saw off from a log matches the optical plate cut out of the crystal perpendicularly to the optical axis (3). Similarly, the 45 plate (1) to the optical axis is sawn off a log.

The presence of birefringence in the wood can be showed that way: if the receiver horn is turned round 90, there will not be any reception. Then the 50 mm plate (2) is inserted between the antennas . If the direction of the layers (optical axis) exactly coincides with the plane of vibration of one of the horns, the signal will not appear. The wave in the wood will be ordinary or extraordinary. If the plate (2) is turned round 45 and with such orientation we place it between the horns, the signal will be significant that testifies the birefringence appearance. 

In optics a quarter-wave plate is used for transformation of linearly polarized wave to circularly polarized wave, and it is sized in millimeters. For SHF waves the wooden quarter-wave plate is a segment of a board with thickness of about 65 mm. This comment is about the obviousness of lecture demonstrations with the wood.

Linearly polarized wave is directed onto the plate. If the vector E is set at 45 with respect to the optical axis, there will be the wave with circular polarization behind the plate. Since the refractive index depends on the different types of wood and air humidity, it is reasonable to compose such plate from two wedges. It is useful to have two pairs of such wedges and to select a quarter-wave plate and a half-wave plate.

An original students practical work on characterization of wood refraction has been performed at our subdepartment (*). For SHF waves with length of 32 mm on the installation working by the Young diagram, the characterization was being measured for the pine. The value received: for an ordinary wave it is 1.35 1.38; for an extraordinary wave 1.47 1.50. Consequently, the wood is a positive crystal.

(*) Mikhaylichenko Y.P. Double refraction of SHF waves in wood. Tomsk SU. 1986. P. 13 (in russian)

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