BIPRISM, FRESNEL'S, an obtuse angled prism used for the production of interference fringes. The prism is set up so that the line joining the obtuse angle to the source of light (in the form of a slit) is normal to the base, and the edge of the obtuse angle is parallel to the slit, this angle lying between the source and the base. Two virtual images of the source, very close to gether, are formed, and the biprism is thus equivalent to two mirrors inclined so that they produce interference. (See LIGHT.)