The linear fringes in this experiment are attributed to interference of secondary waves emanating from the slit. Actually the fringes are due to the surface pattern of the fluid layer present on the bulge middle bulge portion that joins bases of both the prisms in the biprism. One biprism placed in front of a pinhole made in an acrylic sheet and illuminated with Sodium light. Fig-5a shows the fringes with one biprism, Fig-5b shows the fringes with two biprisms kept at an angle in front of the pinhole and Fig-5c shows the fringes with three biprisms placed in front of the pinhole.
Fig-5c shows the fringes with three biprisms placed in front of the pinhole. When two biprisms are placed in the form of a cross in front of the pinhole, a cross fringe pattern appears as shown in Fig-5d.
When the pin hole is illuminated with white light with single biprism, the fringe pattern in Fig-5e is obtained.