frange de Becke

CONT: [...] un moyen d'évaluer les indices relatifs, au microscope en plaque mince; on envoie en incidence à l'aide du condenseur un faisceau non pas parallèle, mais convergent; à la sortie, la lumière se concentre du côté le plus réfringent, en un faisceau légèrement oblique [...]; d'où un liseré lumineux, la «frange de Becke».

CONT: The Becke Line test is a technique used to measure the relative refractive index of two materials. The Becke line is a bright line observed close to the boundary between two grains in plane polarised light. If the microscope focus is raised, increasing the distance between the objective lens and the sample, the Becke line will move into the material with the higher refractive index.