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# Teaching the Nature of Light: New Ways to Demonstrate Interference and Diffraction

During 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 observed straight-line paths of light, he concluded that light 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. (Of course, it fell to Einstein to explain how the photoelectric effect is evidence of the particle nature of light. But that is a topic for another day.)

In his experiment, Young studied patterns produced on a screen when light passed through either one or two slits. From there, he was able to discern which features of the pattern arise from the interaction of the light with the single slit and which arise from the double slits. Today, we know these patterns to be results of interference and diffraction, two key concepts in the nature of light. We can help you teach these important ideas to your students in an exciting and engaging way.

One simple, but fun, approach to measure the wavelength of laser light is with a metal machinist’s ruler. This classic experiment, first developed by laser co-inventor Arthur Schawlow, seems impossible until you see that the experiment makes use of interference between light reflected by nearby marks on the ruler scale. See his article from American Journal of Physics 33, 922 (1965) for details on this memorable experiment that requires little more than a laser and a ruler. For a more detailed experiment that also incorporates diffraction into the mix, we’ve got you covered with the Vernier Diffraction Apparatus.

With the Diffraction Apparatus, students can easily create, view, and measure diffraction and interference patterns. Slits are made by depositing metal film on glass, creating extraordinarily clean slits with fully opaque blocking areas. These high-precision slits cast clean diffraction and interference patterns, ideal for quantitative matching of intensity vs. position predictions. The included Red Diffraction Laser provides a clean monochromatic light source. With our apparatus, you get the entire intensity profile as well as the minimum and maximum intensity locations.

Data collection using the Diffraction Apparatus is performed by choosing a slit, directing the laser through the slit, and choosing an entrance aperture for the light sensor. Both the light sensor and position sensor connect to your interface. Move the light sensor by hand over a 150 mm distance and trace the diffraction pattern.

We also offer two complementing experiments that utilize the Diffraction Apparatus: “Diffraction” and “Interference.” These experiments, found in our Advanced Physics with Vernier — Beyond Mechanics lab book, help students explore diffraction and interference by using Young’s double-slit experiment so students can observe the same phenomena he did.

When it comes to light theory, we know that diffraction and interference are concepts that need to be understood. Whether you explore them with simple real-world examples like light on a steel scale, or choose to go into more depth with an actual double-slit experiment, we’re here to help.