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.