The Nature of Light: Just a Wave?

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Students will confront the limitations of the wave model of light.

Big Idea

Electromagnetic phenomenon are complex - sometimes acting like waves, sometimes acting like particles.

Demonstration: Magnetic Field Induces Current

15 minutes

Today I plant a seed for some work we will do later in the unit. In an attempt to address HS-PS2-5, I show my students how electric current can be induced with a wire passing through a magnetic field. As my focus in this unit is clearly on "radiation," the induction aspects of electromagnetics are harder to work in cleanly. Today's demonstration, and the bulk of today's lesson, is meant to help round out the concept of electromagnetics.

I start by asking my students to consider a simple circuit, with no battery. They respond immediately with a resounding "NO!" when asked if any current will flow in this circuit, pointing out the missing source of energy. I then show them a substantial magnet and the same magnet with a loop of wire which terminates in a 100-ohm resistor. I use a Vernier voltage probe and the Vernier Logger Pro software to run a few trials of voltage versus time. While the data is displayed on the board, I move the wire back and forth through the magnetic poles, producing a series of voltage spikes that clearly coincide with the moments the wire passes through the poles. I can change the rate at which I move the wire and the voltage spike responds: small spikes for slow movement, larger spikes for faster movements. To convince students that this is not just noise, I run a trial without moving the wire: no spikes at all result.

I share a few thoughts about how this effect can be amplified and connect this idea to hydro-electric plants and wind turbines. With this background, students are ready for a short lecture on the history of electromagnetic induction and the synthesis of the studies of electricity, magnetism, and, surprisingly, light!

Maxwell's Synthesis

20 minutes

As we begin to consider the inductive aspects of electromagnetics, I wish to provide some historical backdrop to this peculiar aspect of physics. I present a series of slides detailing the remarkable synthesis of electricity and magnetism that James Maxwell achieved in the mid-1800s. The impact of Maxwell's work cannot be over-stated: the development of the modern world can be traced to these breakthroughs.

Though there are many slides in the presentation, I focus on the first six or so, ending with a cursory explanation of the operation of generators (like the one demonstrated at the beginning of class) and motors. The emphasis is on the surprising outcome that electromagnetic waves travel at the "speed of light." 

Students take notes and ask questions during this time. I assure them that the presentation will be made available to them electronically, so the need to write down every word is relieved.

Reading: Double Slit & Duality

15 minutes

I hand out a relatively short but informative reading. It's an excerpt from John Gribbin's book, Schrodinger's Kittens and the Search for Reality. In the prologue, Gribbin lays out the fundamental wave-particle problem for both light and electrons. I preview our reading protocol and urge students to be attentive to the details of the story; we'll begin our discussion by collecting the details first, before trying to interpret or infer.

The reading is not long, but it is densely packed with counter-intuitive ideas. I give students 15 minutes to read and take notes (see "duality of light" pages 1 through 5 provided as resources).

Protocol-driven Discussion: Double Slit and Duality

15 minutes

To frame the conversation about this reading, I use a reading protocol. The protocol requires students to slow down their processing of the text, forcing them to stay "within" the text during the first phase, ensuring that the details of the text can be fully expressed. During the latter stages of the protocol, students are welcome to infer and ask questions but the essential elements of the text must be fleshed out first.

As the discussion begins, I remind students that while they are speaking they have the floor and are responsible for yielding the floor to others. My role is to record thoughts as they come out and to re-direct the conversation, if necessary. Here is a sample response from one of my two sections. Students seem to get the central problem that light seems to travel like a wave but depart and arrive as particles. Many student comments are about the scale at which this duality exists - at what scale do the quantum rules "give way" to "normal" behavior? I assure them that these kinds of questions put them in great company (like Albert Einstein, Erwin Schrodiniger, and many others).

I allow about 15 minutes for the discussion to unfold. I emphasize the first section of the protocol as a way to ensure that the text is well-understood. There can be many questions but I want to make the text the center of the discussion, not student opinions or wild suppositions, a result that can easily happen with a topic like this one. As the discussion comes to a close, I prep students for a short video clip that will help to reinforce previous ideas about light waves.



Video Clip: Color and Interference Patterns

10 minutes

We end the day with a short Youtube clip that demonstrates several key ideas including the way that light can interfere with itself. While today's reading has threatened the idea that light ALWAYS behaves like a wave, I want students to see that the wave model of light is still, at times, the appropriate model to use. The low-key, fun nature of the clip serves as a nice way to end the day, especially after the difficulty of the reading. Given the nature of the clip and the previous work completed today, I allow students to simply sit back and enjoy the video with no expectation about note-taking or the generation of any formal response.