The physical quantities need to be measured in this experiment – radius of the fringe, distance between etalon and the screen. Make the following table –

Table II : The details of the physical quantities to be measured.

1. Spacing between the etalon by measuring radius of the fringes:-
   1. Level the optical rail with the table.
   2. Arrange the experimental setup as shown in the diagram, with the source on the extreme left. Make sure the multiple dots on the screen converge to a single point using the screws in the etalon before placing the lens.
   3. Keep a (collimating) lens before the etalon in such a way that the etalon configuration is at the focal point of the lens.
   4. Place a (focusing) lens to the right of the etalon such that a clear magnified fringe is observed.
   5. After obtaining the fringes, measure the radius of each fringe using the micrometer scale present under the screen and fill Table 1.
   6. Measure the distance between the screen and the etalon. Note this as D.
   7. Find the spacing between the etalon using the equation \(= \frac{nD^2\lambda }{R_{m+n}^2-R_{n}^2}\).
2. Measurement of the demagnification factor
   1. The fringes will converge (or emerge out) from the central node as you change the distancebetween the etalon.
   2. Count the number of fringes converged as you note the amount by which you rotate the screw attached to the etalon configuration and fill Table 2.
   3. Plot the graph between micrometer reading and number of fringes converging, and find the slope.
   4. Find the demagnification factor using the slope of the graph using the equation \(M=\frac{slope}{\frac{\lambda}{2}}\).
3. Measurement of wavelength of light
   1. Obtain the fringes using a light of unknown wavelength
   2. Find the number of fringes converging (or emerging out), with the screw rotation of the etalon configuration and fill Table 3.
   3. Plot the graph between micrometer reading and number of fringes converged and find the slope.
   4. Using the demagnification factor (say M) found from Step B, determine the unknown wavelength of light using \( λ=\frac{slope}{\frac{M}{2}}\).