Activity 1: Diffraction of Light When laser light passes through a small slit, it forms a diffraction pattern of bright
and dark fringes (as shown below). The central bright fringe is wider than the
others. The angle () between the center of the middle bright fringe and the center
of the first dark fringe is related to the width (W) of the slit and the wavelength ()
of the light by sin   W .
y
NOT TO SCALE
Pattern

L
Slit
W
Experiment
First, you will use the diffraction pattern produced by a laser shining through a slit
to determine the laser’s wavelength.
1. Set up the red laser to pass through a single narrow slit and a
diffraction pattern appears on the screen. The screen should be about 1
meter from the slit. Record the width of the slit used below.
2. Tape a piece of paper to the screen and sketch the pattern that appears
on the screen. Mark the middle of the pattern.
3. How does the width of the central bright fringe compare to the widths
of the other bright fringes?
4. Measure the distance between the slit and the screen.
5. Determine the distance (y) between the middle of the central bright
fringe and the middle of the first dark fringe on one side of the pattern
(as shown in the diagram above). It is easier to measure the distance
between the centers for the dark fringes on either side of the central
bright fringe and divide that distance by two.
6. Using trigonometry, calculate the angle  to the middle of the first
dark fringe. (Hint: Sketch what you measured.)
7. Using the width of the slit (W), calculate the wavelength of the laser.
Show all of your work.
8. How does your measurement compare to the known wavelength of the
red laser (650 nm, 1 nm = 10-9 m)? Calculate the percent difference.
9. Qualitatively, how do you expect the single-slit diffraction pattern to
change if slit were narrower? Assume everything else about the setup
remains the same. Explain your reasoning.
10. Qualitatively, how do you expect the single-slit diffraction pattern to
change if a green laser ( = 540 nm) is used instead of a red laser?
Assume everything else about the setup remains the same. Explain
your reasoning.
Second, you will use a diffraction pattern to determine the width of a hair.
Surprisingly, a narrow barrier will produce the same pattern as a narrow opening of
the same width.
1. Take a hair from someone in your group. Stretch the hair and tape it
down onto the plastic lens holder. Mount the holder onto the track.
2. Position the laser so that it shines on the hair and creates a pattern
similar to what you saw for a narrow slit.
3. Measure the distance between the hair and the screen.
4. Determine the distance (y) between the middle of the central bright fringe
and the middle of the first dark fringe on one side of the pattern (as shown
in the diagram above). It is easier to measure the distance between the
centers for the dark fringes on either side of the central bright fringe and
divide that distance by two.
5. Calculate the width of the hair.