Single and Double Refraction

We know when light is allowed to pass through a medium like glass, we have only one refracted beam and this phenomenon of refraction is known as single refraction. But when a beam of ordinary unpolarised light is allowed to pass through a calcite or quartz crystal, there are two refracted beams of light. A calcite crystal is transparent to visible as well as ultraviolet light. The crystals having this property are called doubly refracting or bire fringent and the phenomenon is called double refraction. To illustrate the phenomenon of double refraction, allow a narrow beam of unpolarised light from a point source to pass through a crystal of calcite. Then two images will be seen on the screen. It can also be illustrated by putting a black dot on a sheet of paper and viewing it through a calcite crystal. Two images of the dot will be seen. On rotating the crystal about the incident ray as axis, one image remains stationary while the other rotates round the first image. The stationary image is known as the ordinary image. The second image which rotates round the first image is called extra ordinary image. The refracted rays which produce the ordinary image are called ordinary rays because they obey the ordinary laws of refraction. While the refracted rays which produce the extra ordinary image are called extra ordinary rays because they do not obey the ordinary laws of refraction. Polarization by double refraction depends on the angle of rotation, when the principle plane of the two crystals are parallel to each other, then the two images obtained, are separated by a distance equal to the sum of the displacement produced by each crystal when used separately. At 90 degree rotation, we again have two beams but the images change roles. The ordinary of the first becomes the extra ordinary of the second and vice-versa. At 180 degree rotation, the principle planes of the two crystals are once again parallel to each other, but having optic axis oriented in opposite directions.

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