As long ago as the 17th century, there were two competing models to describe the nature of light.
Isaac Newton believed that light was composed of particles, whereas Christopher Huygens
viewed light as a series of waves. Because Newton was unable to observe the diffraction of light,
he concluded that it could not be wave-like. Thomas Young’s double-slit experiment in the early
19th century provided convincing evidence that supported the wave model of light. This is the
second of two experiments in which you will examine the related phenomena of diffraction and
In this experiment, you will
? Compare and contrast features of the patterns produced on a screen when light from a
laser passes through either one or two slits.
? Discern which features of the pattern arise from the interaction of the light with the single
slit and which arise from the double slits.
? Use the principle of superposition to explain how waves from two sources could interfere
constructively or destructively.
? Use Huygen’s Principle to construct a diagrammatic explanation of how path length
differences for waves originating at different points in the slit give rise to the dark fringes
in a diffraction pattern.
? From experimental parameters, predict the locations of dark fringes in the pattern.
? Collect intensity vs. position data to test your predictions.
Vernier data-collection interface Vernier Optics Expansion Kit
Logger Pro or LabQuest App Vernier Dynamics Track
Vernier Diffraction Apparatus Green Diffraction Laser (optional)
Direct exposure on the eye by a beam of laser light should always be avoided with any laser, no
matter how low the power. If you performed Experiment 19, your instructor may have you skip
the pre-lab investigation and Part 1 of this experiment.
1. Attach the laser at one end of the track so that it faces down the length of the track. Connect
the power supply. Leave the laser off until all parts are in place to avoid accidental
2. Set the diffraction slit assembly to a single slit of width, a = 0.08 mm. Attach the assembly to
the track, with the silver reflective side of the glass plates facing the laser. Position it about
10 cm from the laser assembly.
20 – 2 Advanced Physics with Vernier – Beyond Mechanics
3. Attach the High Sensitivity Light Sensor and the Linear Position Sensor to the other end of
the track, with the light sensor facing the slits.
4. Turn on the laser. Adjust the horizontal laser position to achieve maximum brightness of the
pattern. Adjust the vertical laser position to center the pattern vertically on the entrance
aperture of the light sensor. Use this procedure every time you change the slits.
Figure 1 Laser and slit assembly Figure 2 Light sensor assembly
5. Slide the light sensor assembly so that the pattern from the beam falls on the screen to one
side or the other of the aperture disk (see Figure 2). Observe features of the pattern and
record your observations in your lab notebook.
6. Now, set the slit assembly to a double slit in which the slit width, a = 0.08 mm and the slit
separation, d = 0.25 mm. Align the laser as you did in Step 4.
7. View the pattern that forms as the laser beam passes through the double slit. Record your
observations in your lab notebook, then turn off the laser.
Discuss what features the patterns have in common and the ways they appear to differ. You may
find it helpful to repeat your observations of the single slit and double slit patterns.
Study the similar behavior of water waves using a simulation that can be found at the PhET web
site: http://phet.colorado.edu/en/simulation/wave-interference. The simulation allows you to
examine the patterns resulting when water waves from a dripping faucet pass through one or two
slits in a barrier.
PART 1 SINGLE AND DOUBLE SLIT PATTERNS–A CLOSER LOOK
1. Return to the diffraction apparatus. Examine the pattern obtained when the laser shines
through the single slit with a = 0.08 mm. Using a ruler, measure the distance between the
midpoint of the central bright region and the midpoint of the first dark fringe. Repeat this
measurement for the second dark fringe. Then, measure the width of the central bright region
from dark fringe center to dark fringe center.
2. Now, take a closer look at the pattern obtained when the laser shines on the double slit used
in the Pre-Lab Investigation (a = 0.08 mm, d = 0.25 mm). Using a ruler, measure the distance
Advanced Physics with Vernier – Beyond Mechanics 20 – 3
between the middle of the central bright region and the location where the alternating bright
and dark fringes seem to disappear. Then, measure the width of the central bright region.
EVALUATION OF DATA
1. Compare your measurements for the single- and double-slit patterns. What feature of the
pattern appears to arise from the single slit? What feature appears to arise from the double
slit? Compare your findings with those of other groups.
2. Consider what feature of the single-slit pattern would change if you changed the slit width.
Test your prediction.
3. Consider what feature of the double-slit pattern would change if you used the same slit width
but changed the separation between the slits. Test your prediction.
PART 2 DIFFRACTION AND THE SINGLE SLIT PATTERN
In a typical discussion of how waves from multiple
slits produce an interference pattern, the slits are
treated as point sources of light. To understand how
light passing through a single slit can undergo
interference, review text or web resources regarding
Huygens’ principle as it relates to diffraction.
Imagine that each point along the wave front in a slit
acts as if it were a separate source of “wavelets” that can
interfere with one another (see Figure 3).
To understand why dark fringes occur in the pattern, consider how the path length difference of
waves from two sources within the slit can result in destructive interference when the waves
reach the screen. The small angle approximation allows you to substitute y/D for sin ? in your
calculation of the distance between the middle of the central bright region and the dark fringes to
either side (see Figure 4).