In this experiment you are to observe and make measurements on the diffraction patterns produced by single and double slits. Your General Physics book (Sears, Zemansky, and Young or Resnick and Halliday) will have a good elementary discussion of the phenomenon in the chapters on diffraction. There are several optics books in the library which give more complete discussions of these effects. The book, "Fundamentals of Optics", by Jenkins and White is recommended. Copies of the pertinent parts of chapters 15, 16, and 17 from Jenkins and White will be placed in the lab for local reference.
The slits are made by a process of electro-etching. They are
mounted in 
slide holders. Treat them gently as
they are quite expensive.
There are three sets of slits and combinations of slits.
One slide has only single slits of various widths, a second slide
has only double slits with four combinations of slit width and
spacing, and a third one has a variety of multiple slits.
The diagram below shows the general arrangement of laser, slit, and screen to be used for these measurments. Following the diagram, place a slide in the holder provided and position it so that only one single slit or double slit is uniformly illuminated by the laser beam. No lenses are required. Observe the diffraction pattern on a white paper against the wall a distance L from the slit. You will need to darken the room somewhat in order to see some fine detail clearly.
for small
angles 
Equations (1) and (2) are the theoretical expressions for the
intensity of the light in
the diffraction patterns. They each predict that the intensity
varies with angle 
. Your measurements will be a
series of y positions of the intensity maxima or minima.
In order to compare your results with theory
you need to convert the y values to angles; do this
using the small-angle approximation
given on the diagram above.
For a single slit, measure the positions of 2 or 3 minima
(centers of the dark bands) of its diffraction pattern.
Since the center of the pattern
is not easy to locate precisely, it is recommended
that you measure the distance between corresponding minima
(2y on the diagram) on either side of the central peak,
and then divide by 2. Also you will need to measure the
distance L from the slit to the screen.
Repeat these measurements for at least two of the four
single slits
on the slide. Be sure to record the width of the slit you are
using.
The equation for the light intensity of a single-slit
diffraction pattern is
where b is the slit width and 
is the wave length
of the light. 
is the intensity at the center of the
pattern where 
.
This equation predicts minima (where the intensity I =0) when
and that occurs when 
equals any integer
multiple of 
. Thus setting 
yields
as the condition for minimum intensity.
The first minimum occurs when n=1 and so on.
For each slit, use Eqn. (1) along with your data to calculate the slit width b. Compare your result with the value for b given on the slide. The wavelength of laser light is known to be 632.8 nm.
The double-slit patterns are more complex because they contain both the double-slit and the single-slit pattern together. The theoretical intensity pattern is given by
Notice that this equation is different from the single slit
equation only by the addition of
the 
factor which results from the
double slit. This term is a maximum (brightest spots) when
is an integral multiple of 
. Thus setting
yields 
as the condition
for brightness, where d is the distance between the slits.
Now the single-slit factor involving 
also controls the
intensity, so there will also be minima in locations determined
by the slit width b.
For at least two double-slit combinations measure the positions of the bright, narrowly spaced spots using the same technique used to determine the single-slit minima. Use these measurements along with the known wavelength to determine the slit spacing d. Compare your results with the dimensions given on the slide. In addition, measure the position of the intensity minima which are caused by the single-slit diffraction (these minima are quite widely spaced compared to the double-slit effects). Use the data to determine the slit width b just as was done for the single slit patterns above. Compare the results to the values given on the slide. Notice that a double-slit maximum can fall on a single-slit minimum if d is an integral multiple of b. This results in what is known as a missing order in the double slit pattern.
You may wish to observe some patterns caused by other types of apertures. Multiple slits and holes which are symmetric polygons produce rather nice patterns. Feel free to try a variety of things.