8.1.3 Accurate Fringe Counting

The following techniques can help you make accurate measurements.

1. It's not necessary that your interference pattern be perfectly symmetrical or sharp. As long as you can clearly distinguish the maxima and minima, you can make accurate measurements.
2. It's easy to lose track when counting fringes. The following technique can help. Center the interference patter on the viewing screen using the thumbscrews on the back of the fixed mirror. Select a reference line on the millimeter scale and line it up with the boundary between a maximum and a minimum. Move the micrometer dial until the boundary between the next maximum and minimum reaches the same position as the origina boundary. The fringe pattern should look the same as in the original position. One fringe has gone by.
3. When turning the micrometer dial to count fringes, always turn it one complete revolution before you start counting, then continue turning it in the same direction while counting. This will almost entirely eliminate errors due to backlash in the micrometer movement. Backlash is a slight slippage that always occurs when you reverse the direction of motion in a mechanical system. (Turning the micrometer dial clockwise moves the movable mirror toward the right.)
4. Always take several readings and average them for greater accuracy.
5. The slip ring at the base of the micrometer knob adjusts the tension in the dial. Before making a measurement, be sure the tension is adjusted to give you the best possible control over the mirror movement.
6. Warm up the laser. Many lasers vary in intensity and/or polarization as they warm up. To eliminate any possible fringe or intensity variations, allow the laser to warm up for at least an hour, if possible, before setting up an experiment.