Draw a ray diagram showing position of virtual sources and region of interference in biprism experiment.

Answer: The biprism splits the wavefront from the slit S into two parts which appear to come from two virtual coherent sources S₁ and S₂; interference occurs in the overlapping region on the screen.

Diagram (described in words):

• On the left, a narrow vertical slit S illuminated by monochromatic light.
• A short distance to its right, the biprism, drawn as a thin triangle with its obtuse edge (about 179°) pointing towards the slit and its refracting edge parallel to the slit, placed symmetrically about the axis.
• Rays from S striking the upper half of the biprism are bent downwards, and rays striking the lower half are bent upwards.
• Extend these refracted rays backwards (dotted lines): they meet at two points S₁ (above S) and S₂ (below S), in the same plane as S. These are the virtual sources, separated by a distance \(d\).
• On the right, a screen at distance \(D\) from the plane of S₁S₂. The two refracted beams overlap in a region between them, shown shaded and labelled region of interference; on the screen this region contains alternate bright and dark fringes, with the central bright fringe on the axis.

Labels: S, S₁, S₂, \(d\), biprism, \(D\), region of interference, screen.