Since presentations were cancelled due to bad (possibly icy) weather,
we decided to do a complete experiment to study the diffraction pattern
of laser light after passing through two narrow slits. The "theory group"
worked out a formula (prediction), and the "experimentalists" made
the measurements and plotted them in a spreadsheet program. The theory
uses the knowledge of complex numbers that we talked about at the previous
meeting. "k" in the formula is an abbreviation for (2*pi/lambda), where
lambda is the wavelength of the light.
Here are some pictures of the slits. The slide has 2,3,4 and 5 slits 
we used the double slit because this is the easiest one to calculate.
Some of the pictures were taken by pointing the camera at a microscope.
Each slit is 40 micrometers (microns) wide, and the centers of the slits
are separated by 125 microns.
These black&white photos show the center of the diffraction pattern
at two different distances from the slits, about one meter and about
three meters. The spots look the same at the further distance, except
that the whole pattern is three times bigger. (The camera zoom was
adjusted to make the pictures match in size.)
This is the beautiful data obtained when the detector is working
correctly! The red line comes from the expression derived by the
"theory group," after adjusting the height, width and position to
best match the data. Now that it's clear how well the experiment is
working it would be fun to get more data, covering several of the
peaks.
These two plots show the expected diffraction/interference pattern calculated
with a more complete theory that takes into account that the two slits each
have a nonnegligible width. The horizontal scale is the angle that the
light leaves the slits in milliradians. In other words, the horizontal scale
corresponds to +/ 50 mm on the screen, if it is placed one meter from the
slots. The vertical scale (light intensity) for the figure on the left is just
big enough to hold the center peak; in the plot on the right the vertical scale
is increased by a factor of 20 to show better the small peaks. Note that all of
the peaks have the same width  they only vary in height (light intensity).
