Determination of the order
To determine the order of the interference colour, the rays are made to cross another anisotropic material (compensator) after leaving the crystal to try to compensate for the retardation introduced by the mineral.
The compensators are wedge-shaped, so that the resulting retardation gradually increases with the increasing thickness. They are usually made of quartz and for this reason they are called quartz wedges. They are mounted on supports in such a way that the fast component vibrates according to the longer side of the wedge and the slower one vibrates in the shortest direction (figura 1, movie). They are inserted in a slot in the microscope, at an angle of 45º to the directions of vibration of the nicols (polariser and analyser).
As the light passes through the compensator, this introduces retardations which are superimposed on the retardation introduced by the mineral. The results will be very different according to whether the fast and slow components of the mineral coincide with those of the compensator (fast with fast and slow with slow) or not (fast-slow and slow-fast), figure 2.
What will happen in each case?
Compensators producing additive effects
When the fast and slow components of the compensator coincide with those of the mineral (fast with fast and slow with slow), this will lead to additive effects. The advantage which the fast wave gains over the slow one during its path through the mineral will increase when passing through the compensator. The final retardation will therefore increase and, as a result, the interference colour will be higher.
These compensating plates are normally used, as their name suggests, to compensate for or nullify the retardations introduced by the minerals. Obviously, in the position fast-fast and slow-slow this objective is not achieved.
How can the retardations be compensated for?
When the fast and slow components of the compensator are opposite to those of the mineral (fast with slow and slow with fast) this leads to subtractive effects. The advantage that the fast wave gains over the slow one during its path through the mineral is lost when passing through the compensator, on finding that the slow component of the the compensator vibrates in this direction. The final retardation is reduced and consequently the resulting interference colour goes down in value.
This is the correct position for compensating plates; if we wish to nullify the retardation introduced by a mineral, we must simply progressively increase the thickness of the compensator.
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