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. Both models could explain reflection and refraction, but the phenomena of diffraction and interference could be more easily explained by Huygens’ wave model. In the early 19th century, Thomas Young’s double-slit experiment provided evidence that supported the wave nature of light. This is the first of two experiments that examine the related phenomena of diffraction and interference. You will first compare the patterns that are produced when laser light passes through one or two slits and then strikes a screen. You will then focus your attention on the analysis of the double-slit interference pattern.


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 a diagrammatic explanation of how path length differences for light passing through the two slits give rise to bright and dark fringes in the pattern.
  • From experimental parameters, predict the spacing between bright (or dark) fringes in the pattern.
  • Collect intensity vs. position data to test your predictions.