The direction of movement of the Becke Line is determined by lowering the stage with the Becke Line always moving into the material with the higher refractive index. The Becke Line can be considered to form from a cone of light that extends upwards from the edge of the mineral grain.
Becke line can be considered to represent a cone of light propagating up from the edges of the mineral.
If nmin < noil, the cone converges above the mineral.
If nmin > noil, the cone diverges above the mineral.
By changing focus the movement of the Becke line
can be observed.
If focus is sharp, such that the grain boundaries are clear the Becke line will coincide with the grain boundary.
Increasing the distance between the sample and objective, i.e. lower stage, light at the top of the sample is in focus, the Becke line appears:
Becke line will always move towards the material of higher RI upon lowering the stage.
A series of three photographs showing a grain of orthoclase:
1. Photo 1 – The grain is in focus, with the Becke line lying at the grain boundary.
2. Photo 2 – The stage is raised up, such that the grain boundary is out of focus, but the Becke line is visible inside the grain.
Photo 3 – The stage is lowered, the grain boundary is out of focus, and the Becke line is visible outside the grain.
When the RI of the mineral and the RI of the mounting material are equal, the Becke line splits into two lines, a blue line and an orange line. In order to see the Becke line the microscope is slightly out of focus, the grain appears fuzzy, and the two Becke lines are visible. The blue line lies outside the grain and the orange line lies inside the grain. As the stage is raised or lowered the two lines will shift through the grain boundary to lie inside and outside the grain, respectively.
It is not possible to get an accurate determination of the refractive index of a mineral in thin section, but the RI can be bracket the index for an unknown mineral by comparison or the unknown mineral with a mineral whose RI is known.
Comparisons can be made with:
Becke lines form at mineral-epoxy, mineral-mineral boundaries and are interpreted just as with grain mounts, they always move into higher RI material when the stage is lowered.