Becke line is a band of light seen along the mineral grain boundary under the PPL arrangement when the microscope is slightly out of focus. Depending on the way the microscope is focused, the Becke line may lie within or outside the grain boundary.
The Becke line method is used to determine the relative refractive indices of two minerals. This is performed by lowering the microscope stage or increasing the focal distance (increasing the distance between the section and the objective). When this is done the Becke line will appear to move towards the material with the higher refractive index. This method is used to compare the refractive indices of two minerals or to compare the mineral with a medium with known refractive index such as Canada Balsam or Epoxy glue.
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