Today, I had hoped to spend most of class engaging in peer instruction of thin-film conceptual questions. Instead, we spent most of class discussing the electromagnetism and electromagnetic induction exam which I passed back today. Students had a lot of questions beyond the broad feedback that I wanted to share with the class. Always hard to strike a balance between spending time in class going over an exam about which most students don’t have any questions and giving students an opportunity in class to ask questions. Today, given the overall performance on the exam, I leaned towards to later.
Today was a half-day of instruction followed by a half-day institute. Not much to do with classes that short other than take a practice quiz on thin-film interference.
No photo of the quiz or the class; so, I’ll share a photo of a board designed by students on the Huskie Robotics team that plugs into the MXP on the new roboRIO controllers for the FIRST Robotics Competition. This board exposes all sorts of additional functionality through the MXP. Such a cool project and fantastic to see the fabricated boards in hand!
Today students prepared and presented whiteboards of solution to physical optics problems. In the past, I’ve always done geometric optics before physical optics. This year, I swapped the order to better align to the text. I was concerned that the students would find physical optics too abstract without some experience with geometric optics, but they seem to be doing just fine.
This groups’ whiteboard was clearly presented. I like how they claim their diagram is to scale!
Today, we calculated the thickness of a sheet of paper based on the measurements made in yesterday’s activity. It was very challenging to accurately count the number of fringes on the glass slides. In addition, I don’t have a micrometer to check the thickness of the paper. So, the only confirmation was that the calculated thickness was reasonable. I’ll need to acquire a micrometer for next year.
Some students are drawn to the thin-film interference equations in our textbook like a moth to a flame. I’m trying to convince them that they don’t need any equations to solve these types of problems. If they consider if the reflected waves as phase shifted and then determine if constructive or destructive interference is needed, they can reason through all of these problems.
Today’s activity was to determine the thickness of a sheet of paper by placing it between the ends of two glass slides and observing the interference pattern on the glass slides. Based on my experience trying this activity ahead of time, I was concerned it would be challenging to make the observations, and it was. Some groups had more success than others. We ran out of time before any group finished the calculations. We’ll do that tomorrow.
I need to make several improvements for next year. I suspect that a sodium lamp would work better than a variety of CFL bulbs in different colors, which we used. I also found that the binder clip that I used to clamp the two slides together do so too tightly and distorted the slides resulting in curved interference lines rather than a series of parallel lines. I tried using a rubber band instead, but it didn’t hold the slides tightly enough. Maybe I need thicker glass slides.
Anyone else do a lab like this? What equipment do you use? Any suggestions? Thanks!
Today, we spent most of class on discussing conceptual questions concerning the diffraction and interference of light. However, I started by introducing the small angle approximation which is helpful when solving many double-slit problems. Students were surprisingly unaccepting of the validity of this approximation, I suppose to the credit of their math teachers. I wanted to show them how good this approximation was, so I fired up Desmos and made a quick graph:
A couple students say they will still solve these problems “long-hand” as a matter of principle.
Today, we started the Physical Optics unit in AP Physics 2. I demonstrated various examples of diffraction and interference phenomenon with this apparatus which holds a plate with several different types of single and double slits and diffraction gratings. I’ve also mounted a thin wire to demonstrate refraction around an object.
While, students find the demonstrations interesting, I didn’t help them appreciate the incredible precise measurements you can make by leveraging these phenomena. I think next year, I need to take a different approach and develop a paradigm lab to start this unit.