Fresnel’s biprism is an optical device used to produce two coherent virtual sources from a single monochromatic source. It is used to demonstrate the interference of light and to determine the wavelength of monochromatic light.
A Fresnel biprism consists of two thin prisms joined at their bases. It is equivalent to two thin prisms placed base-to-base. Structurally, the biprism acts as a single piece of optical glass with one obtuse angle (approx. 179°) and two small base angles (approx. 0.5°).
When light from a narrow slit falls on the biprism:
- The upper half deviates the light in one direction.
- The lower half deviates the light in the opposite direction.
- The rays appear to come from two virtual images, and , of the original source .
Thus, and act as two coherent sources and produce interference fringes on the screen.