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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

Fresnel’s biprism consist of two prisms of very small angles joined base to base. In
practice, a thin glass plate is taken and one of its faces is ground and polished till a
prism is formed.
When a light ray is incident on an ordinary prism, the ray is bent through an angle
called the angle of deviation. 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.
A Biprism, in the same way creates two virtual sources S1 and S2. These two virtual
sources are coherent because they are images of the same source S created by
refraction.
When Light is incident on the biprism. Light passing through the lower section is
refracted up, while light going into the top section is refracted down, forming a region
where the beams interfere. This creates two virtual sources S1 and S2, with a
separation � between them