Lab C4: The Double-Slit Diffraction Pattern)
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In this experiment the intensity pattern from diffraction of light by a pair of slits is measured in detail and analyzed. The diffractin pattern can be fit using a non-linear least-squares fitting program. A semi-automated computer data acquisiton can be assembled to collect the data.
A Figure 1. Diffraction of light by two parallel slits. II. Theory. The intensity pattern from double-slit diffraction is given by where 1 is the angle from the axis of symmetry, and and Here a is the slit width, d is the slit separation, and is the wavelength of the light. Here we have used the small-angle approximation, sin 1 . 1 .
Procedure
  1. Preliminary measurement. Find a suitable double-slit. The one used in the lower-division labs is very good. There may be others which work OK in the case in the back of room 125. Find a He-Ne laser with a screw hole in the bottom, and mount it on a rod in a lens holder, on the optical track. Project the pattern on the wall. Can you make out the "double-slit" and "single-slit" effects? Experiment with the pattern by changing from 1 to 2 to n slits. What do you see?

    Now set up the Pasco luxmeter and optical fiber, with the receiving end of the fiber mounted on a traveling stage, on the optical track. After making the Pasco luxmeter work, take a quick scan of the diffraction pattern hile while you figure out the controls and scales. After you are comfortable with the equipment and it seems to be working reproducibly, make a detailed scan of the pattern. You may want to use a digital voltmeter on the analog output to record light readings. Plot the data roughly by hand, in your lab book. Does it look good? Using this plot, make estimates of the slit width and separation, with errors. B. Calculation of the Intensity. WRite a Fortran program to evaluate the theoretical intensity formula above. Write a data file of (1, I) points. Use MONGO to graph the theory on top of your data points. Dois it seem to fit? C. Linear Least-squares Fit. Now Project Lab Double-Slit Diffraction