Fresnel Biprism : A freshnel biprism is a thin double prism placed base to base and have very small refracting angle ( ). This is equivalent to a single prism with one of its angle nearly 179° and other two of each. Here the interference is observed by the division of wave front. Monochromatic light through a narrow slit S falls on biprism ABC , which divides it into two components. One of these component is refracted from portion AC of biprism and appears to come from S₁ where the other one refracted through portion BC appears to come from S₂. Thus S₁ and S₂ act as two virtual coherent sources formed from the original source. Light waves arising from S₁ and S₂ interfere in the shaded region and interference fringes are formed which can be observed by placing a screen MN .

If d is the separation between the virtual sources S₁and S₂, Z₁ is separation between source S and biprism and Z₂ is the separation between biprism and the screen then

                                             Z₁ + Z₂ = D

        and fringe width                            (11.1.1)

The separation ‘d' between the source can be measured experimently and is described in the following section.