The Becke line test is a technique in

optical mineralogy Optical mineralogy is the study of minerals and Rock (geology), rocks by measuring their optics, optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrog ...

that helps determine the relative

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

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 to identify Rock (geology), rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descr ...

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 Canada balsam, also called Canada turpentine or balsam of fir, is the oleoresin of the balsam fir tree (''Abies balsamea'') of boreal North America. The resin, dissolved in essential oils, is a viscous, sticky, colourless or yellowish liquid t ...

or an oil of known refractive index (

oil immersion In optical microscope, light microscopy, oil immersion is a technique used to increase the Angular resolution#Definition of terms, resolving power of a microscope. This is achieved by immersing both the Objective (optics), objective lens and the ...

). When permanently mounted to a slide under a cover slip, the mounting medium is normally chosen to have the same refractive index as

(''n''=1.55) to avoid confusion when comparing with previously made slides. If a different mounting medium is used, its 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).

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Further reading