Observations in convergent light

If we examine the sections of an-isotropic minerals under a convergent polarized light, the ”interference figures” can be seen. In order to observe these figures, we need to make some pre-adjustments in the polarizing microscope.

  1. Polars are crossed.
  2. Condenser lens is used below the stage to produce a converging beam.
  3. A high power objective is used.
  4. A Bertrand lens is introduced between the eyepiece and the analyser. In case wheh Bertrand lens is not used, interference figures are seen by removing the eyepiece.

Interference figures are observed in those sections of anisotropic minerals which are: (i) normal to an optic axis of uniaxial or biaxial minerals, and (ii) normal to bisectrix in biaxial minerals.

(1). Uniaxial interference figure:

Fig: 01

Uniaxial interference figures consist of a black cross with its arem parallel to the cross hairs and a series of concentric coloured rings. The colours of these concentric rings correspond to that of the spectrum. The black cross is formed where the vibration directions of the E-ray and O-ray coincide with the vibration directions of the polarizer and analyser. Thus the two arms of this cross indicate the vibration directions of the E-ray and O-ray.

Determination of Optic Sign: In an optic axis of interference figure, the E-ray vibrates radially and the O-ray tengentially. With the help of an accessory plate, the optic sign of a crystal can easily be determined. Let Mica plate be inserted at 45° to the cross hairs over an optic axis figure. Here two possibilities arise.

  1. In negative minerals, in which the O-ray is slow, the mica plate will reinforce the interference colours in the SE and NW quadrants causing them to shift towards the center. In the other pair of quadrant, subtraction takes place and the colours move away from the center. Two black spots are also formed in these quadrant near the centre of the black cross.
  2. In positive minerls, the above effect is reversed as shown in following figure.

    Fig: 02

In the optic axis figures, which show low order interference colours, the gypsum plates is used and in those which show high order interference colours, the quartz wedge is used.

(2). Biaxial Interference Figure:

Although interference figures can be observed on any section of biaxial crystals, the most important are those obtained an sections normal to the acute bisectrix.


Fig: 03

Acute Bisectrix Figures: This figure is observed on a crystal section cut normal to the acute bisectrix of a biaxial mineral. If the optic axial angle is moderate, say less than 45° , both the optic axes emerge within the field of view of the microscope.

The typical acute bisectrix figrues are shown in following figure. When the optic axial plane lies parallel to the East-west cross-hair  (plane of vibration of the polarizer). a figure similar to that of following figure (a) is observed and when the optic axial plane makes 45° angle with the cross-hair, the figure like that shown in fig: 03 (B) is observed.

When the plane of optic axis is parallel to the vibration direction of the polarizer, the biaxial interference figure shows a black cross. The horizontal bar of this cross is thinner and better defined than the verticle one. At two points on the horizontal bar, there are concentric dark elliptical curves. The outer curves fuse to form a double curve.

Fig: 04

On rotating the microscope stage, the black cross breaks into two hyperbolas. These hyperbolas are called “isogyres”. They show maximum separation at 45° position and each becomes narrowest at the point of emergence of optic axes. The isogyres are always convex towards the acute bisectrix.

The optic signs of biaxial crystals can be determined on acute bisectrix figures witht he help of accessory plates. The determination of the optic sign of a negative crystal with the gypsum plate is illustrated in figure: 04.


Other related posts:

  1. Optical Properties of Minerals
  2. Refractive Index of Minerals
  3. Isotropic and An-isotropic minerals
  4. Double Refraction of light in minerals 
  5. Uni-axial Minerals
  6. Bi-axial Minerals
  7. Optical Indicatrix of Minerals
  8. Polarized Light Microscope
  9. Examination of Minerals under Polarized Light Microscope
  10. Examination of minerals in cross polar
  11. Extinction in minerals and Types of extinction
  12. Accessory plates used with a polarizing microscope
  13. Sign of Elongation

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