The transmitted part is then partially reflected at the glass plate. It passes back into the lens (beam B) to create two phase coherent beams with different optical paths, arising from the air gap between the lens and the glass plate.
Experimental Method.
The apparatus was first checked for quality. This involved wiping the lens and glasses surfaces free of dust and fingerprints which could have otherwise affected the fringe pattern.
A sodium vapur lamp was used because this is a good monochromatic source of light. A strip of plane glass under the microscope and lens (see diagram 2) allowed partial reflection of the sodium lamp's light whilst also allowing light into the microscope.
The microscope's eyepiece had a cross-hair in the middle of the image which was used to aim at the interference rings. The central dark fringe was pointed at first. The microscope was focused to make the rings as clear as possible. The lens could then be moved on a slider with increments as small as 0.01mm.
The procedure for this part entailed measuring the diameters of the dark and bright fringes. The diameters were measured to increase the accuracy (despite the use of radii in formulae).
Taking readings proved difficult as reading measurements as small as 1/2mm in dim light put great strain on one's eyes. A magnifying glass was used to aid measurment taking. Despite the 0.01mm accuracy of the sliders, error much larger than this will have arisen because of the difficulty found in taking readings. We took readings moving away from the centre (of the ring pattern) and then came back across it to take measurements for the other side of the circles. Re-measuring the centre's position gave us a good idea of the inaccuracy involved both in moving the sliders as well as judging the central position. Similarly great care had to be taken not to jog the apparatus as this would have adversely affected results.