This is a sample of the data we took from the He-Ne Laser. Notice that it is not a perfect sine wave - this is because we could not coordinate the sampling rate of the data aquisition with the frequency with which fringes passed the photo multiplier tube.
This is a Fourier spectrum of that data (with the dominant peak at 632.8 nm):
The following are samples of data taken with the Sodium light as our source. The beat patterns are obvious
Here is a Fourier spectrum of the data with dominant peaks around 588.9 nm and 589.5 nm (the spectral lines of sodium).
Using the known wavelength of the He-Ne laser (632.8 nm), we could find the wavelength of the Sodium light. Our calculations:
