The Becke line appears as a band of light along the boundary between two media with different refractive indices. A Becke line might be observed between the mounting medium and crystal, or between adjacent crystals. It is caused by differences of the refractive index between the mountant and the object. To enhance the edge between a crystal and its surrounding mounting medium or between two adjacent crystals plane polarised light with a narrow parallel axial or oblique light beam can be used. Plane polarised light can be achieved with a light microscope with inserted polariser but no analyser. Narrow parallel axial light is achieved by closing the condenser aperture slightly. When the condenser is slightly horizontally decentered the object is illuminated with an oblique light beam. Whether the Becke line appears inside or outside the boundary of a crystal depends on the focus position. If the crystal has a higher refractive index than the mounting medium the Becke line will appear inside the crystal in the above focus position. Moving below the focus plane the Becke line will surround the crystal.
If the crystal has a lower refractive index than the mounting medium the effect is reversed. Above the focus plane the Becke line appears outside the crystal, below the plane of focus the Becke line is visible inside the crystal. Therefore, the Becke line can be used to get information about the relation of refractive indices of different media.