Determination of order: step by step


As indicated in the previous screens, order is determined by using compensating plates to value the retardation introduced by the mineral. By placing the compensator in a subtractive position (fast-slow slow-fast), we nullify the retardation created by the waves when passing through the mineral.

In order to nullify the retardation, what we will have to do is:

firstly, ensure that the directions of vibration of the mineral and the auxiliary plate coincide (both at 45º to the directions of the nicols) and...

secondly, check that the slow and fast components of the mineral and those of the compensator are inverse (fast with slow and slow with fast).

We must observe the colour of the mineral while gradually introducing the compensator and note the changes in the interference colour which appear. The wedge is inserted until a grey-black colour is obtained, which represents the moment at which the retardation introduced by the mineral has been annulled due to the retardation introduced by the compensating wedge. The number of red-violet mixtures through which it passes in the succession of colours must be noted, and the order of the colour will be the number of red-violets plus one.

If, when introducing the wedge, the colours which appear show an ascending sequence, towards increasingly higher interference colours (i.e. from yellow to orange to red etc...), this means that we are not in the right position (fast-fast and slow-slow) and the mineral must simply be turned round 90º so that the slow component of the mineral will then be compensated by the fast one of the wedge and vice versa. Once this position is achieved, we will introduce the wedge again and the colours will show a descending sequence (i.e. from red to orange to yellow etc...) until we get to grey which represents total compensation (figure 1).

The procedure to follow is shown first outside the microscope, with only two polaroid plates and two strips of anisotropic material, one of them cut with a variable thickness in the form of steps (movie 1). The second video shows how the order of the interference colour is determined under the microscope.

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