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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.

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.

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.

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.

What some people call the "vertex angle" refers to the triangular cross-section. If you think of a prism as a 3D polyhedron object this is misleading. The Fresnel biprism has a very flat isosceles triangular cross-section, so the internal angle along the middle edge is close to 180 degrees. In 3D this is one edge of the prism.

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.

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.