The physical quantities need to be measured in this experiment –Fringe width (length), separation between the virtual sources and screen (length), separation between magnified image of the two virtual sources (\(d_1\)), separation between the diminished image of two virtual sources (\(d_2\)) Make the following table –

1. Setting up the experiment:-
   1. Level the optical rail with the table.
   2. Set the Sodium vapour lamp is on the left most part of the optic bench.
   3. Keep the mounted rectangular slit is close to the small opening of the vapour lamp.
   4. Place the mounted Fresnel bi prism to the right side of the rectangular slit at a distance of 10 to 20 cms (this value is to be kept unchanged for one reading of fringe width).
   5. Put the Eyepiece on the rightmost side of the optic bench. Check if all the mounts are aligned and height-adjusted, such that the fringe is visible.
   6. Align the tangents screws are to that ensure the edge of the bi-prism and the slit are on one plane.
2. Finding the distance between two virtual sources using displacement method:-
   1. Place the mounted lens between the eyepiece and the bi-prism.
   2. Keep the eyepiece at a large distance from the lens(about 4 times the lens’ focal length)
   3. Move the lens to observe a diminished image of the two slits.
   4. Note the gap distance between the diminished image of the two virtual slits is as \(d_2\)
   5. Move the lens to observe the magnified image of the two slits.
   6. Note the gap distance between the magnified image of the two virtual slits is as \(d_1\)
3. Measuring fringe width:-
   1. Remove the lens from the optical rail.
   2. Bring the eyepiece suitably near the bi-prism to observe a clear fringe pattern.
   3. Measure the width of 20 bright fringes using the micrometer on the eyepiece.
   4. Divide the reading by 20 to get the width between two consecutive bright fringes and note it as \(\beta\). This can be done with dark fringes as well.