Becke method [after the Austrian scientist F. Becke], one of the variants of the immersion method of measuring the refractive index n of a solid substance. The substance under study, finely crushed, is placed in a drop of liquid and observed under a microscope At the boundary of two media with different n, owing to the phenomena of interference and of total internal reflection, a thin bright band appears — the Becke band. When the tube of the microscope is raised this band moves towards the substance with the larger n, and when it is lowered — towards the substance with the smaller n. When the n of the liquid and of the substance are equal, the Becke band disappears. Using a set of liquids with known n, the n of the solid substance is determined.