Laser Interference - Fresnel's Biprism
Interference of laser beam using Fresnel's biprism.
Laser cart with Fresnel's biprism on double cross-carriage mount, -10cm concave lens, and screen.
A Fresnel biprism is a glass prism with a very large angle. A 5 cm focal length convex lens focuses the laser light on the biprism. The laser beam enters the flat side and exits at the tip of the symmetric triangle with the beam split by the two halves of the prism, which are very nearly parallel. The exiting halves of the beam are refracted so that they overlap and interfere, producing a series of fringes. The 10 cm focal length concave lens aids in expanding the fringes so they can be seen on a nearby screen, as seen in the photo above.
J. J. Veit and D. J. Solarek, Interference fringes using a Fresnel biprism and a laser, TPT 13, 413-414 (1975).