Tomsk State University. Physical Department.
Laboratory of training physics experiment
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Conical   Refraction          

Conical Refraction is a unique phenomenon with great history which has not been examined completely yet. The experiments are difficult because of small crystal sizes. That is why formerly the event had been observed under a microscope. Aragonite was the only one available crystal from the first Lloyds observations in 1832 up to Ramans works who observed this phenomenon on the artificial naphthalene crystal under a microscope in 1941.

Our experiments were performed on rhombic sulfur monocrystals in 1978. We applied the method of light focusing on the crystal surface that enabled to refuse the traditional diaphragm to observe internal conical refraction. As result, we succeeded in obtaining large images for the phenomenon observation in a lecture room.

The crystal plate is slightly inclined to show a transformation of the rings

This is the photo of two rings of the internal conical refraction. The rhombic sulfur plate is illuminated by a narrow focused beam by He-Ne laser and slightly inclined, and the second surface is projected onto the screen by the objective. Sulfur is a soft and brittle material therefore the scratches on its surface are seen. The rings diameters are about 4 mm on the plate and 20 cm on the auditorium screen due to projection magnification.

The existence of the dark Poggendorff ring proves that it is impossible to create in reality a strict parallel light beam of finite intensity. Inner and outer rings of light are formed by the rays that fall at different angles to the binormal, and therefore they are the result of birefringence.  

In white light by using light bulbs the rings of internal refraction

are projected onto a translucent matted screen.

Image changes by increasing the beam width

... narrow beam
Illumination by wide beam

A bright spot is observed in the center of the rings of internal refraction as a result of focusing action of plane-parallel plate.

Change of the image as a result of inclination of the plate
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With increasing inclination of the crystal plate to the falling light beam the rings deformation takes place. Gradually they are transformed into two arcs, and then into two beams which could be obtained by birefringence.

See more details in our publication

 

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