Question

With a neat labelled ray diagram explain the use of Fresnel's biprism to obtain two coherent sources.

Answer 1

Fresnel biprism: A biprism is a prism with a vertex angle of nearly 180°. It can be considered to be made up of two prisms with very small refracting angles ranging from 30c to 1°, joined at their bases. In the experimental arrangement, the refracting edge of the biprism is kept parallel to the length of the slit. Monochromatic light from a source is made to pass through a narrow slit S as shown in Figure and fall on the biprism.

The two halves of the biprism form virtual images S₁ and S₂. These are coherent sources obtained from a single secondary source S. The two waves coming from S₁ and S₂ interfere and form interference fringes like that in Young’s double slit experiment in the shaded region shown in the figure.