Lerner Physics

So, I'm finding the end of the year coming, and I'm actually in a good place. We're going to finish early. We'll have lots of time to review.

I don't think I've ever had that thought before. 

We talked about atomic emission spectra. Of course, I didn't have the tubes ready today (I'll have them tomorrow) so we didn't get to do a lab with it. Still, I could show pictures of the sun's spectrum and its dark lines.

We figured out how to model atomic transitions. The best question was about how this model is similar to and how it different from the model of the photoelectric effect. We talked a lot about the differences of the atoms and what happens to the electron. 

Today was very teacher-centric. I don't know how I feel about this. For one, this is difficult stuff to conceptualize, and for me to be there, introducing parts of it here and there and letting them try some problems about it, works better. They have some pretty strong models on the electric force and on waves that we can build on. I also feel shakier about this unit, which I abbreviate PMAM (the particle model of all matter), than any other unit this year.

I know I could make it more modeling-friendly. I could give them results to experiments and have them come up with explanations. But what those experiments look like, and what makes the data seem real rather than just words on a sheet of paper, isn't clear to me yet. 

It's okay that I'm not done with this unit. I can see the improvements I can make in previous years. It's not like this year isn't successful. The derivation of the Bohr atom went smoother this year than ever before. But there's still more to do to perfect my practice.

We tried to find Planck's constant using LEDs. We got numbers in the ballpark, but they weren't great. I think I've learned how to make it work better next time. 

It's very interesting to see groups work through variables and units for their equations. We still like using J and E interchangeably for energy. We still aren't exactly sure how to find the units of the slope. These are skills I must emphasize more next year. 

Then we started talking about how much like particle photons really are? Like, do they have momentum? And, if they do, then do other particles, like the electron, have any wave properties?

We drew some atomic LOL diagrams after the long test on the wave model of light. Most of us like those diagrams, but there are a few who don't. I like the system with the photon going in and the electron coming out, even though we ARE keeping the energy of the electron in our system. I'll try to modify the LOL next year to make that more obvious.

What can we explain with light as a ray, as a wave, and as a particle? Let's whiteboard!

So which one is light, really?

Now I get to blow their minds. None of these models works perfectly. We have to make a kludgy wave-particle hybrid called a photon.

It ends in tears.

But then I made it all better when I said we can energy bar charts (LOL diagrams!) to explain the photoelectric effect. I'll show some whiteboards for that tomorrow.

I need to build a lab for this. But, when I don't have a lab, it always seems that PHeT is there. And I'd use the PHeT simulation for this every year just because it makes it so much easier to visualize.

I always start the photoelectric effect by talking about Einstein's Nobel Prize, and the papers he wrote in 1905. I misspoke today, thinking that Einstein's work on the specific heat of metals was published that year. It wasn't, but his work on Brownian motion was that year instead. That makes that year even more amazing in my opinion.

We then experimented with what changed about the current as we changed the intensity and the wavelength of the incident light. We also investigated what happens when we change the voltage applied across the gap. Our results seemed weird until we came up with the quantum hypothesis.

Yes, we played more with diffraction gratings, and we saw great patterns by just using one white light source (see above for three enthusiastic students' pictures of the diffraction). We also played with an electric rotator and strobe light bought long before I ever taught at this school. It's fun to play with persistence of vision. And we did clean up the mess of the back of my room.

But, more importantly, we talked about academic honesty and standards-based grading. Academic honesty is becoming an issue at our school. Students are driven to do well, feel pressure to do well, and will often do almost anything to get a good grade. But, my students said, with my grading system, the focus was on understanding and not doing. I then asked them for help about how I should tweak the grading scale for AP Physics 1 next year, and, with their help, we came up with this outline:

There are four levels, and a student progresses through the levels sequentially.

Level 1: Earned if a student was in class and did the labs and participated in some way in going over the problems

Level 2: Earned by getting a "Quick Quiz" 100% correct. I'll write many versions over the summer. They're designed to be feedback, to see if a student understands the major ideas of the standard.

Level 3: Earned by doing well on a problem in the wild. I will no longer tell them a reassessment is "for this standard." The students must find a problem in the wild, on a longer assessment, and use that standard correctly, along with other ideas.

Level 4: Earned by showing mastery, either through particularly elegant solutions, using the idea over time, or solving difficult problems. This level is the least fleshed out at this point.

I also imagine you can't pass the class without earning at least level 1 on each standard and can't get above a D+ without earning at least level 2 on each standard. At least that's where I am now. Of course, any and all feedback is welcomed for this idea, which I remember seeing in some form on Matt Owen's blog.

We finally had a sunny day where the day wasn't packed with new material, so we went outside to use lenses and mirrors to focus parallel light rays to a focal point. And to burn things.

We then practiced doing single-slit, double-slit, and thin film questions. It was a low-key day.

What causes the colors we see when we look at two plates of glass that are separated by a small air gap?

Before we could explain that, we had to talk about color. I wanted a short discussion about how we see color so we'd have some ability to say whether the colors of the two plates of glass looked more like rainbow colors or subtractive colors. We had so many questions about color. Color is so interesting, with tetrachromancy and bee vision being obvious tangents. We'd love to talk about it more, but I hurried everyone to looking at the plates of glass. So many students are gone Thursday and Friday for (another) robotics competition that I wanted to make sure all the new concepts were introduced today.

We saw the colors, and they were cool. They did not look like rainbow colors.

We then explained them by looking for a path length difference. But I didn't let them figure it out enough. Next year, I should make them do more of the thinking. 

We started by working on lots of problems in class. I didn't give any homework at all last night. If we rush, we could finish the unit on Thursday, but why? Break is Friday, and if we take it slowly, we'll finish the unit on Friday and be ready to start the photon model of light refreshed after break. We'll need it; that model is weird.

We whiteboarded lots of diffraction grating and double slit questions, and it seemed that we were confused about when to use one equation and when another. It's a good idea to talk about the assumptions of each of the slit equations. The assumptions behind the slit equation are subtle, and I don't think I realized that until I was challenged so much this year. Thanks, class.

The last two questions we whiteboarded were about what we'd expect to see from one slit or from a small pinhole. We made predictions and then, of course, went to lab to see if we were right. We weren't. But when we talked about what was happening, it started to make sense.

Since so many students will be gone tomorrow, we'll learn the last part of the wave model tomorrow and then practice everything Thursday and Friday.