What is Becke line (Becke line method) – in forensic science

A method used in microscopic analysis to determine the relative differences in refractive index of two adjacent media, such as a particle and the surrounding mounting media. The Becke line appears as a bright halo of light surrounding a specimen that is immersed in a liquid. When the refractive index (ri) of a specimen is the same as the refractive index of the liquid, the Becke line vanishes. This phenomenon can be further exploited by moving the specimen relative to the objective of the microscope. The apparent direction of motion of the line, either into the liquid or into the specimen, is useful in determining how the refractive indices relate to each other. In forensic science, the Becke line method is used in the analysis of particulates such as glass, minerals, and fibers.