Procedure for performing the real lab:
To find the wavelength of the laser source:
- The laser beam must strike at the center of the movable mirror and should be reflected directly back into the laser aperture.
- Adjust the position of the beam splitter so that the beam is reflected to the fixed mirror.
- Adjust the angle of beam splitter to be 45 degrees. There will be two sets of bright spots on the screen, one set from the fixed mirror and another from the movable mirror.
- Adjust the angle of the beam splitter to make the two sets of spots as close together as possible.
- With the screws on the back of the adjustable mirror, adjust the mirror’s tilt until the two sets of spots on the screen coincide.
- Expand the laser beam slowly by rotating the collimating lens in front of the laser.
- Align the laser with the interferometer and make certain that the fringes are moving when the micrometer screw is turned.
- Mark a point on the screen and note the micrometer reading.
- As the screw is moved, the fringes begin to displace. Count the number of fringes N that move past the mark (either inward or outward). To avoid the effects of backlash in the micrometer screw, turn the micrometer handle one full turn before starting the count.
- Note the micrometer readings at the beginning and end of the count. Calculate the distance d' the mirror is moved, according to the beginning and ending micrometer readings. Repeat the procedure several times. Average the readings.
- With a known wavelength laser, use d = Nλ/2 to calculate the actual distance moved. The calibration constant of the interferometer is then k= d/d'. All subsequent distance measurements with the micrometer should be multiplied by the calibration constant k. Ideally, k would be exactly 1, but factors such as wear and thermal expansion can cause it to vary.
- Once the calibration constant is known, if the laser source has an unknown wavelength, it can be calculated with the same equation.
Procedure for performing the simulator:
- Using the combo box Choose laser, select the desired laser source.
- Switch ON the laser source using Power On button.
- A blurred fringe pattern will appear on the screen, shown in the zoomed view at the upper right corner of the simulator. Make the fringes sharp using the slider Adjust Mirror. Once a coarse adjustment has been made with the slider, the right and left arrow keys on the keyboard provide fine control of this adjustment.
- Using the slider Adjust micrometer, slowly change the micrometer distance. The fringes will displace. The distance moved for a fixed number of fringes can be noted from the values displayed above the slider. The light blue up and down arrows provided in the simulator can also be used to move the micrometer. A third way to move the micrometer, useful due to the fine control it provides, is with the left and right arrow keys on the keyboard. They must first be activated by dragging the slider with the mouse. (In the simulator, the micrometer reading is offset by a fixed amount, so that it is not possible to set d = 0 to get a single large dark spot.)
- The wavelength of the laser source can be calculated using the equation (2) from the theory page. (The calibration constant k for the simulator is exactly 1.)
- The Reset button resets the whole experimental arrangement to its default (starting) configuration.
- The Show results button displays the results after doing the experiment.
- The experiment can be repeated for different laser sources and for glass plates of different thickness.
Observations and calculations:
Least Count = ..... cm
Calibration constant of the apparatus= .....
No: of fringes, N =.....
Distance moved for N fringes, d =...... cm.
Then, λ= 2Δd/N
The wavelength of the given laser source = .......... nm.