The Nature of Light: Wave Properties
Lesson 5 of 17
Objective: Students will leverage key algebraic equations and some important wave concepts to explore the wave characteristics of light.
The bulk of today's lesson comprises two large sections of time: first a section of teacher-driven content followed by a section of student practice and application. Although this is not my favorite mode of instruction, given the exploratory nature of the past few lessons it is perhaps unavoidable: if students are to process the ideas of electromagnetic waves any further, they need some grist for the mill. By enhancing student understanding of the electromagnetic spectrum, we build toward several of the NGSS Performance Expectations, including HS-PS4-1, HS-PS4-3, and HS-PS4-4.
I begin the day with a review of the comparison of waves. I highlight the similarities and differences between mechanical and electromagnetic waves. Students have this as a handout and add in additional notes as they see fit. With the introduction of "constructive and destructive interference," I show the following slide on the board. By moving the waves with respect to one another, I illustrate the differences that occur as the waves are more or less in sync with one another. I show constructive interference with the two waves perfectly synchronized and destructive interference when they are perfectly de-synchronized.
As a follow-up, I shift the waves to some random relationship to one another and ask students to consider where, on the axes shown, the two waves would momentarily cancel. This student response shows that point by means of a green line that is positioned to mark that moment. I repeat this for 2-3 arrangements of the waves as a quick check for understanding.
I also take some time to review the electromagnetic spectrum and its organization by frequency. Students will receive this page as part of handout later in class, so I advise them to simply follow along with a short series of thoughts. I stress the similarities among the bands with the only distinguishing characteristic being frequency. Finally, I share some thoughts about Max Planck's realization that energy of electromagnetic waves depends linearly on frequency. While this leads to some thoughts about wave-particle duality - as it is the energy of a photon that depends upon the frequency of the wave - I steer away from any deep thoughts about that topic. I want my students, today, to be able to work with the energy relationship and allow the deeper, more counter-intuitive, ideas to emerge over time.
I provide students with a set of problems that ask them to demonstrate some previous skills (sketching waves, writing wave equations, etc.) and some new skills (identifying destructive interference, relating frequencies with energy, etc.). I encourage students to work collaboratively. Furthermore, because I want students to get to the newer skills quickly, I advise them to do the first problem then skip ahead to the third one for just that purpose. A solution set is provided for convenience.
Here, students are about ten minutes into their work on this problem set:
I circulate to help students in small groups or one-on-one:
With about fifteen minutes to go in class, I ask students to turn their attention to the board where I show a short clip demonstrating the interference pattern that is the result of light passing through a double-slit apparatus. I want the idea of "light as a wave" to be the dominant idea that students leave with.
To prepare students for the idea that light could interfere with itself, I show the following image of water wave interference. I take a few moments to highlight the dark streaks and the bright segments as areas of destructive and constructive interference, respectively.
I then show the following clip of a recreation of Thomas Young's famous double slit experiment, using some very simple materials and sunlight. Students enjoy seeing the naive thoughts of adults and the engaging graphics and visuals. The interference pattern is easily understood as a result of light's wave behavior. I end the class with some additional comments about the clarity that can be achieved in the interference pattern if one substitutes a single color for the full-spectrum light used as the source in the experiment. This is easily achieved with an inexpensive laser pointer and, at a later point in this unit, students will replicate this experiment.