Student Guide

Particle-Wave Duality

What models best explain the behavior of light? Does light behave like a wave, a particle, neither, or both?

What models best explain the behavior of light? Does light behave like a wave, a particle, neither, or both?

  • Laser pointer, any kind (red lasers work best because of its larger wavelength)
  • Thin copper filament/strand. Many wires are made with copper strands. Cut a piece of a cable that you don’t mind discarding. Remove the rubber and separate the intertwined pieces of copper. Get a single strand, depending on your laser pointer 3 inches will suffice.
  • Electrical tape
  • Paper, notebook or whiteboard to write the observations
  • Note: You can find other DIY setups that use old DVD discs, aluminum foil, or even cardboard paper. We have found this the easiest and most reliable form of doing it.
  • A demonstration can be found here: https://www.youtube.com/watch?v=kKdaRJ3vAmA
Intro

Scientists use experiments to develop an understanding of the world around us. We propose hypotheses and design experiments to test them or we ask questions like “why do objects/things always fall to the ground?”. Often the technology that we can use to do our experiments changes and evolves, which allows us to refine our experiments and therefore expand our knowledge and understanding of the world. What is light? Have you ever wondered how light travels, whether it moves in a straight line or if it spreads in multiple ways? Two of the questions scientists considered in the early 1700s were “what is light?” and “how does it travel?” For many years, there were different theories, and physicists designed many experiments to test the different hypotheses. Today you are going to follow in their footsteps to answer these questions yourself.

After reading the introduction, what is your essential question or objective for this activity?

Before the Experiment
  • What do they expect to see when a particle goes through an obstacle?

    What do they expect to see when a particle goes through an obstacle such as small holes in a paper? What would they expect to see if a wave went through a piece of paper? What do they expect light would look like?

    1. Think about this for yourself
    2. Discuss with your group
    3. Draw group models on the white board
    4. Discuss with the class
Setting Up
  • Let students carefully shine the laser onto a wall or a screen

    Once predictions are complete, let students carefully shine the laser onto a wall or a screen. Ask them to write down their observations and how what they saw compares to models of particles (thinking about sand grains as particles) and waves without obstacles.

  • Next, ask students to build their double slit diffraction set up

    Next, ask students to build their double slit diffraction set up.

  • Take the piece of copper filament/strand and place it across the laser pointer

    Take the piece of copper filament/strand and place it across the laser pointer, where the beam comes out.

  • Use electrical tape to secure the copper strand

    Use electrical tape to secure the copper strand to the laser pointer on the top and bottom. Then add extra tape to the side to make a smaller aperture for the beam to go through.

During the Experiment
  • Shine the laser light onto the screen or wall

    Shine the laser light onto the screen or wall. You should see a horizontal pattern of dark and bright lines or dots that represent the crests and troughs of the interference of the light as it passess through the double slits. Patterns are easier to see if the lights are off and the wall or screen is flat with no edges that interfere with the light as it passes through the double slit.

    WARNING: Be careful to not shine the laser on people’s faces or eyes.


  • Figures below are two examples of what you should see on the screens/wall

    Figures below are two examples of what you should see on the screens/wall. The top picture patterns were created with the laser pointer set up shown above. The bottom picture shows a double slit pattern from a physics lab.

  • Draw what they see and compare to their predictions

    Draw what they see and compare to their predictions and also to what they expect particles or waves will be like.

Conclusion
  • How do your observations explain the need for a theory of wave-particle duality of light?

    How do your observations explain the need for a theory of wave-particle duality of light?

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