AIR WEDGE METHOD -THICKNESS OF A WIRE

The Two plane glass plates are taken. They are cleaned thoroughly. They are held in contact at one edge. At the opposite edge, a thin paper is inserted between the glass plates. the air-wedge is formed between the lower surface of upper and upper surface of lower glass plate.
The glass plate system is kept on the platform of a travelling microscope. The light from a sodium vapour lamp is rendered parallel with a condensing lens and is made incident on a plane glass sheet held over the wedge at an angle of 450 with the horizontal. The light falling on the sheet is partially reflected which in turn incident normally on the air wedge. Adjusting the arrangement properly, the microscope field of view is made bright to the maximum extent.
The microscope is moved vertically up and down till parallel fringes are visible which are located on the surface of the air film.
By moving the microscope in a horizontal direction, the vertical cross-wire of the microscope is coincided with one of the (n-th) fring in the pattern. It's position is noted down. The microscope is moved further along the length of the air film counting the number of dark fringes. After counting fringes and coinciding the vertical cross wire of the microscope with now the N-th fringe, position of that N-th fringe is noted. The measurements are repeated three times and are recorded in table.
The length of the air film is measured as the distance between the line of contact of glass plates to the inner edge of the Wire. the measurement is done using travelling microscope.