What is Frensel's Biprism.??
Fresnel's Biprism is an optical device which basically consists of two prisms joined at their base to result in a single 'lens like' prism. It was designed by Augustin Fresnel for the sole purpose of performing interference experiments and studying wave nature of light. The diagram below elucidates...
One angle of the prism is obtuse, close to 180o while the other two are equal and acute, close to 30o each.
The two sides of the prism, upper and lower refract incoming light and act as two slits. The refracted light rays interfere and form a pattern on the screen.