We looked at diffraction gratings again. Since about 1/3 of my class was gone Thursday and Friday for various competitions (mostly robotics), we built on Thursday and Friday today in class to make sure everyone is at the same place. So we used the better diffraction gratings today and different laser colors. (I love blue lasers. So cool.) We also whiteboarded some of the fundamental questions about how double slit interference works. Today was a good day, but not a day packed with material. That's fine; we should finish the unit this week, right before going on Spring break, so we can start Modern Physics when we get back.
We don't feel comfortable with slits yet. It's difficult to visualize the path length difference, and I think I spent too much time lecturing. I'll fix it next week.
Still, we did get to see how two slits work in the real world. And, when we went to the diffraction grating, which is just an infinite number of slits to a physicist, we could predict what would happen. I'll take that.
It's interesting to watch the patterns, both good and bad, we've fallen into as a class. The patterns are mostly good; don't get me wrong. But I like hearing the shared language, the statements of "this fits with our model" or "wait, is this like.." as we tried to find connections between our models. It's a great feeling when you feel you've changed how students conceptualize science.
We played with double slits today. We had a small class, with lots of students out for robotics competitions and Football Hall of Fame field trips. So we made some qualitative observations, and they fit with our model of two sources we talked about yesterday. One student was very surprised. "Wait, so that model actually works?"
Yes. Yes it does.
"It means that light is a wave," said another student.
"Well, this model works for now," I replied. "Don't be so sure."
Then we derived the path length difference equation for a double slit. I did it too soon. I should have waited until we had a better understanding of path length difference, with simpler questions, before going into the complicated double slit derivation.
Most of today was a test. But, the best part was whiteboarding our answers to the double-slit questions. We got confused about what would happen when the two sources were brought closer together. We couldn't really communicate, so I got the great idea to get two tape measures to act as the two sources. We could see the path length difference, and when the two people moved closer together, we could see how we had to change where we were measuring the path length difference to keep the "one wavelength" difference (of 25 centimeters). I recommend this demonstration.
To me, it was a reconfirmation. If a student asks a question, more and more, my gut says "let's try it out" rather than "let's reason it out" or "let me tell you." It's the best change in my teacher I've ever made.
We whiteboarded our results from the diffraction experience yesterday and then saw what would happen if you had two very small openings. It seemed to make sense with sound, and then we tried it with light. It worked! The photo above is after we talked about what would happen at various points in the diagram—would we get constructive or destructive interference?
We had exchange students from Takatori, Japan today in class. It was an honor to show the students what my class was like. I hope they had fun.
I planned the first part of the lesson to be something we could experience, so, even if they didn't understand all the words, they could get something out of it. I broke out the polarizers. We looked at the many things through a polarizer—the sky, glare, our old calculators, even the older iPhones. We saw some real differences. Then, with two polarizers, we came up with an idea of why polarizers give us evidence that light is a transverse wave.
Then we used this simulation we can use on a Chromebook to watch how waves deal with openings. We definitely saw a pattern with small versus large openings, so we ended up talking about diffraction.
I think I'm liking geometric proofs in physics! There's something powerful about these derivations. I think that model-building isn't always just trying it out in lab; sometimes it's about using our model and mathematics to come to new conclusions which we, of course, must check against reality.
I got the animation above from PHYSCLIPS, which I had never heard of before today. It was great for visualizing the wavefronts and rays, but then, when I right-clicked and hit rewind, it paused the animation so I could do some geometric magic.
Next year, I have to figure out how to make the kids do more of the derivations. But I've done proofs like this a few times this year, and I like it.
We practiced our new understanding of reflection and refraction for the rest of the day.
I remember reading that all of the AP Physics 1 and 2 free response questions could be turned into a laboratory experience. That happened today.
We spent some time talking about how to approach last year's AP Physics 2 question on reflection and refraction. Then a student asked to see a demonstration. Easy. I pulled out a laser and a tank of water. He took a picture of the setup, as you can see above. It became much clearer to my students what reflection and refraction was happening.
I also learned, by doing this question, I need to talk about how, when reaching any boundary, some light reflects and some refracts. What a good point to emphasize. I know how to incorporate it into the questions I already ask.
We finished the ray model of light today, and we started talking about wavefronts. We modeled wavefronts as kids holding hands and walking together. We saw what happened as the wavefronts went from a faster medium to a slower medium.
We drew lots of ray diagrams today. Lots of them. Other than calling lenses mirrors, we seem pretty comfortable with them. Once you know lenses, though, and go back to mirrors, it gets confusing.
We also seemed confused about what side of a lens you need to be on to see the image. We tried it in lab and then talked about why that makes sense.
We then started practicing some of the old AP Physics B questions on reflection and refraction and then last year's AP Physics 2 question on reflection and refraction. We'll talk about how to think about AP Physics 1 and 2 questions tomorrow. It's the first time I've spent a significant chunk of time about what the exam looks like.
We had a fire drill, and I grabbed the large converging mirror. I don't want to stop with the ray model of light.
Today, though, felt a little repetitive. Oh, look, principal rays that are parallel to the axis. Oh, look, real, inverted, smaller than the object images that are located between f and 2f. We've seen this before. The analogy between mirrors and lenses are strong.
But the applications of lenses are more interesting. Every time I talk about lenses, I end up talking about how eyeglasses work. They're interesting, and I can explain people's prescriptions.
I talked a lot today. Tomorrow, everyone will talk. We'll need to think through the implications of the ray model.