LAB 9
Interference & Diffraction Experience
OBJECTIVES
In this lab you are going to work with a single slit apparatus to try and learn how
measurements of diffraction patterns can be used to verify and then extend your
knowledge of the objects that create them.
PROCEDURE
Part 1: Review of Single and Double-slit Diffraction
a. Set up a double slit interference experiment and find the wavelength of your diode
laser. Be sure to write out your procedure and document what data you took and how
it helped you determine the wavelength.
b. Now change to a single slit of width a = 0.08 mm. Measure the location of the
diffraction minima and use your calculated laser wavelength to verify the known slit
width.
c. Replace the Single-Slit Set with the Multiple-Slit Set. Use the Double-Slit with slit
width a = 0.04 mm and slit separation d = 0.25 mm. Predict and measure the (i)
width and (ii) number of fringes inside the central maxima. Be sure to write out your
procedure and document what data you took and how it helped you determine the slit
width.
d. Repeat part (c) for slit width a = 0.08 mm and slit separation d = 0.25 mm.
e. List some factors that affect the diffraction pattern. Be sure to think about things like
the fringe spacing, fringe brightness, pattern orientation, etc.
Part 2: Designing an Experiment
a. Now design an experiment to determine the size and shape of the diffraction pattern
produced by shining a laser at a small square opening. Think about each measured
variable in your experiment and determine which of the measured variables will have
more uncertainty than all the others, or will they all be roughly equally important?
(Hint: consider the percent uncertainty of each.)
b. How would you use the information above to design an experiment to measure the
size of a square hole?
c. Now run your experiment using the Single Slit Disk set to the square hole. Did you
learn something from attempting the experiment that caused you to modify your
design? Analyze your results to find a best guess and an uncertainty for the diameter
of the hair. Be sure to write out your procedure and document what data you took
and how it helped you determine the width of the hair.
Part 3: Identifying the Shape of an Unknown Object
Analyze the diffraction pattern of the unknown object. Your job is to determine the
shape, size, spacing, etc. of the unknown object as best you can. Make sure you give
some reasoning for all your “guesses.” (Hint: You might find it useful to go back and look
at a double slit interference pattern and notice the superposition of the
double and single slit patterns.)
This is a picture of an x-ray diffraction pattern that Rosalind
Franklin took in 1951. She used it to determine the structure of a
particular crystal.