The Becke line test is a technique in optical mineralogy that helps determine the relative

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...

of two materials. It is done by lowering the stage (increasing the focal distance) of the

petrographic microscope A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology wh ...

and observing which direction the light appears to move. This movement will always go into the material of higher refractive index. This index is determined by comparing two minerals directly, or comparing a mineral to a reference material such as Canada Balsam or an oil of known refractive index ( oil immersion). When permanently mounted to a slide under a cover slip, the mounting medium is normally chosen to have the same refractive index as Canada Balsam (''n''=1.55) to avoid confusion when comparing with previously made slides. If a different mounting medium is used, it's refractive index should be recorded ''on the slide'', to avoid loss of the information. Media used for impregnating a specimen before sectioning (either for mechanical strength, or to pick out porosity with a contrasting colour) are also usually chosen with the same 1.55 refractive index. If a specimen is mounted without a cover slip - for microprobe analysis, backscattered electron microscopy, reflected light microscopy ... - then an immersion oil can be chosen with whatever refractive index is desired for the study. The method was developed by Friedrich Johann Karl Becke (1855–1931).

References

Further reading