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

Fresnel's biprism splits light from a single slit into two virtual coherent sources; interference fringes appear only in the region where the two refracted beams overlap.

Construction/ray path (described, since no image is printed in the source paper):

• A narrow vertical slit $S$, illuminated by a monochromatic source, sends a divergent cylindrical wavefront onto a biprism (a very-obtuse-angle prism, effectively two thin prisms joined base to base).
• The upper half of the biprism refracts the light downward and the lower half refracts it upward, so that after the biprism the emergent rays appear to originate from two virtual images of $S$, labelled $S_1$ (above the axis) and $S_2$ (below the axis), separated by a small distance $d$. These act as two coherent virtual sources, since both derive from the same original source $S$.
• Beyond the biprism, the two sets of refracted rays diverge from $S_1$ and $S_2$ respectively and spread out and overlap in a lens-shaped (roughly diamond/eye-shaped) region — this overlap region widens as one moves further from the biprism towards the screen.
• Only within this overlap (shaded) region do the two coherent wavefronts superpose and produce observable alternating bright and dark interference fringes on a screen placed there; outside the overlap region, light from only one virtual source falls, and no fringes are seen (only uniform illumination from that single beam). …