Conoscopy of Uniaxial Minerals.(2).
Let's now examine the behavior of uniaxial crystals cut perpendicular to the optic axis in monochromatic light. The crystal used is a calcite coming from a Berek compensator (see picture below). With this device, the crystal can be rotated to move the pole of the optic axis towards the periphery of the figure so that more extinction rings can be seen. On all the figures below, a small rule has been included to easily monitor the position of the extinction rings according to the wavelength.
As seen above, an extinction will occur if the condition d.(n' - n") = k.l is met. The retardation (d.(n' - n") ) increases from the pole of the optic axis (point zero of the rule) towards the periphery and will reach the value l at smaller angle for shorter wavelengths. The first ring appears at 3.3 cm from the center at 420 nm and at 4.3 cm at 680 nm. 10 rings are visible at 420 nm and only 6 at 680 nm. In white light, the shift of the extinction rings produces color fringes. The first extinction being in the violet, removes the violet light from the white and thus appears yellow. This effect can be easily seen by the use of a medium band green-orange filter: the first ring is dark with green and orange borders (see below).
As we move away from the center, we can see a mixing of the extinction orders: for instance at 8.3 cm, we find sixth extinction ring for the violet 420 nm and only 4th ring for the orange 600 nm light. Due to this effect, the colors of the rings in white light become less and less sharp away from center.
If the thickness of the section is small and its birefringence is small, the angle to reach the first extinction ring can be so large than it occurs out of the field of the figure. In this case, only the dark cross (isogyre) can be seen with no interference colors. See for instance a quartz plate.
420 nm |
450 nm |
505 nm |
520 nm |
578 nm |
600 nm |
628 nm |
680 nm |
Orange-green medium band filter. The first extinction is for green light so only orange light can pass through the filter. The inner part of the rings is thus orange. The contrary is true for outer part of the rings which appears green. |
White light. First ring from center is yellow because first dark ring occurs in violet image.
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Berek compensator used as target crystal to produce conoscopic images above. This mechanical device allows tilting of the crystal by a rotation of the knurled knob. This motion moves the conoscopic figure off center. |
