A Fresnel Biprism is a narrow, base-to-base double prism with an extremely tiny refracting angle.
This is the same as a single prism with one angle of about 179° and the other two angles of 0.5 degrees each.
The splitting of the wavefront reveals the interference
Monochromatic light passes through a small slit S and is split into two components by biprism.
One of these components seems to originate from S1 and is refracted from the upper section of the biprism, while the other appears to come from S2 and is refracted from the bottom portion.
As a result, S1 and S2 function as two virtual coherent sources that were created from the original source. In the darkened region, light waves from S1 and S2 collide, causing interference.
Fresnel biprism is an experiment that shows interference by the division of wavefront