How can the apex angle of a Fresnel biprism be equal to $180^\circ$?

Question

I am learning about Fresnel biprism. I wanted to know about biprism, and found out on the internet that it is a triangular prism with apex angle $180^\circ$. But I am unable to interpret $180^\circ$. The above image is what I found of triangular prism but I am unable to see the $180^\circ$ apex angle.

Normally, I see a triangular face with somewhat $60^\circ$ and not $180^\circ$. For $180^\circ$ it would have been almost a straight line, can someone please point out where is the apex angle related to $180^\circ$? It would be better if you could share a clear picture with apex angle of $180^\circ$.

Answer 1

What some people call the "vertex angle" refers to what you see in the triangular cross-section; the Fresnel biprism has a very flat isosceles triangular cross-section.

If you think of a prism as a 3D polyhedron the term is misleading, as the vertex of the cross section corresponds to a long edge of the polyhedron. In a Fresnel biprism the internal angle along the middle edge is close to 180 degrees. This shows in the cross-section as the very flat apex.

In practice the very sharp edges on either side are often cut off or truncated, so what you get is a similarly flat pentagonal prism, as in the sketch below.

Answer 2

It is not exactly 180 degrees, but a bit less --- 179 degrees,say. Do a Google search for Fresnel Biprism and you will see many pictures.