Labs to explore

We decided to do a pedagogical experiment this week in my general physics 2 course.  I wanted to see if I could have the lab be more like the ones in my sound and music course, where students used lab to further explore ideas in order to shore up their understanding of the topics. The idea was to introduce and work with topics in class on Monday and Wednesday and then tell them that the quiz on Friday would be about that material (that’s not new) and that they could use whatever notes and observations they made in lab (on Thursday) during the quiz (the new thing).

DISCLAIMER: I’m writing this post on the day after the quiz, but two days before I grade the quiz. I’m doing that because I’m still excited about this experiment now, and I might not be so much after grading the quiz.

The topic was diffraction and resolution. We explored ways to add phasors, who all have the same phase relationship, to achieve a dark spot. For example, for 4 phasors you have a clockwise square, a counter-clockwise square, and what we called “back and forth and back and forth.” There’s only 3 ways to add to zero with four phasors and that helps explain why a quadruple slit has two sub-peaks between the main peaks in the diffraction pattern. For resolution we talked about how far apart things have to be for us to discern that there’s actually two things there. I felt that both classes went pretty well. We talked about finite numbers of phasors and infinite numbers (like in a single slit) and multiple infinities (like in realistic multiple slits). I was especially happy with using the infinite phasor argument to explain why a single slit would lead to dark spots.

In lab my colleagues made available a plate with 25 different slit configurations and two different color laser pointers. There were also boards to act as screens. I told the students that they should definitely investigate all 25 with both colors and record whatever they thought would help them on the quiz.  One thing I specifically encouraged was to look for “missing orders” where the single slit dark prediction coincides with the multiple slit bright prediction.

The quiz consisted of these three prompts:

1. Describe your hunt for “missing orders.”
2. What logistical challenges were there in lab?
3. Considering the theory we developed in class this week, what other investigations would you like to do in lab if logistical/financial/temporal concerns could be ignored?

The standards that were going to be assessed on the quiz were:

• I can calculate the location of peaks for multiple slits and zeros for single slits using phasor ideas.
• I can find where missing orders would be and discuss imaging resolution.

When I started the quiz I said “hopefully your answers to the prompts will help me identify whether you’ve mastered these standards.” I did that because I was a little worried that they’d talk about things that happened in lab that wouldn’t really have much to do with the standards. For the logistical one I was hoping for issues that could be connected to the theory of diffraction. For example something like “we realized that putting the screen further away would have helped identify the spacing of bright spots, but we could only move it so far away on our table.” I also hoped that the last prompt would solicit comments about resolution, since the lab was really mostly about the multiple slit diffraction standard.

So, we’ll see. I’ll grade them Monday and maybe I’ll augment this post at that point. For the moment I think it went pretty well. I talked with some students after lab and asked what they thought. The comments ranged from “it was easier than most labs because I didn’t have to do every single measurement” to “I didn’t really know what to do in lab.” I think I’m ok with both of those extremes.

Your thoughts? Here are some prompts for you:

• I’m in this class and I thought this was a good lab experience. What I really liked was . . .
• I’m in this class and I didn’t like this at all. It would have been better if we had . . .
• Why didn’t you give them some 2D diffraction targets?
• Here’s a great lab setup to get at the resolution standard . . .
• What would the lab have been like if you hadn’t made this change?
• Here’s what I predict will make you crabby when you grade the quizzes . . .
• Here’s what I predict will make you laugh when you grade the quizzes . . .
• Here’s what I predict will make you not update this post after you grade the quizzes . . .
• I like your phasor approach. Can you use it to explain how bright the sub-peaks will be?