There are two prominent methods to obtain interference fringes of light; using Michelson Interferometer and Newton’s rings apparatus. A third method used in which directly vertical fringes are obtained is using a Fresnals Bi Prism. This Bi Prism is specially designed whose acute angles are of the order of 30secs. The third angle will naturally be near 180 degrees. The apparatus is called Bi Prism Assembly.
The experiment involves an optical bench made of 132cms long 3″ Stainless Steel rods on which a narrow slit, Bi Prism holder, a double convex lens and a micrometer eye peice is used. This Optical Bench is supported on 2 rugged cast iron supports. The positions of the uprights is noted on a stainless steel square rod. Different parts of the Bi Prism Assembly include an adjustable Sharp Slit provided on the 1st upright. The 2nd upright carries the Bi-Prism on a special mount and can be easily replaced by any other Bi-Prism. The Bi-Prism holder can be gradually rotated by a slow motion screw to align in the vertical plane to make it exactly parallel to the slit for sharp fringes. 3rd upright carries the double convex lens used to find the distance between the two vertical images of the slit. 4th and the last upright carries the eye-piece provided with cross line graticule mounted on a slide. This slide is moved by a micrometer screw and the distance moved is read on the screw guage upto 0.01mm. The slit is mounted on a fixed upright and the other 3 uprights are provided with slow motions to remove the lateral shift. In our Bi Prism assembly, the height of all the uprights is factory adjusted which need no adjustment by the user making this instrument very student friendly and trouble free.