Fresnel's biprism consist of two prisms of very small angles joined base to base.As a result, the ray emerging out of the prism appear to 
have emanated from a virtual source located a small distance from the real source.These two virtual 
sources are coherent because they are images of the same source S created by 
refraction.A Biprism, in the same way creates two virtual sources S1 and S2.When Light is incident on the biprism.between them