## Day 120: Reflection and Refraction Redux

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.

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## Day 119: Drawing Lots of Lens Diagrams

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.

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## Day 118: Fire Drill with the Converging Mirror

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.

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## Day 117: Jumping Around from Refraction to Lenses

I got a little ahead of myself today, but I think it'll all work out okay in the end.

The question my guest blogger was so interested in was what would happen if you only had half a mirror. Would it still work as a mirror? Would you still get an image? I wanted them to see it in real life, but I know it works better with lenses.

So, after we got through some of the refraction homework, we went back into lab, to look at what happens when you cover parts of a lens. They were shocked. A great picture of before and after is shown above.

While we were in lab, we graphed 1/di vs. 1/do for a converging lens. This might have been too much. We hadn't even gone over all of refraction yet. But the graph was so nice, and we were already in lab, and it seemed to make sense at the time.

Hopefully we can make sense of all of it tomorrow. We'll see.

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## Day 116: Guest Blogger

Below is written by a student. I took a personal day. I'm so proud that they debated the answer!

Today's test was a little longer than expected. By the time everyone had finished, there was only twenty minutes left of class to begin whiteboarding. For the first three questions, the class reached a general consensus, using object distance and focal distance to solve for image distance and height. Things took a turn for the last question, as I'm guessing you predicted. We entered little schools of debate, motioning and scribbling to figure out what image, if any, would arise from reflection off of half a concave lens. Yet, as we reached the pinnacle of the debate, we were kicked out by the substitute and left with a number of literal and figurative question marks swimming around the question. We are hoping for some answers (okay not answers, more like insight) on Monday.

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## Day 115: Refraction

We didn't start today with whiteboarding. Usually we do. I wonder, is it a bad idea to start with whiteboarding so often? Is it a good way to make sure we're all on the same page?

We started instead with a lab. We used the three pin method not to find out how reflection works but what happens when the light enters a new medium. We found the relationship between the angle of incidence and the angle of refraction. It wasn't linear, and it doesn't make sense to square an angle in radians, so we tried some mathematical transformations that make sense for angles.

I'm off tomorrow, so one of my students has promised to be the guest blogger tomorrow. They have a test and some whiteboarding to do. I won't be there to supervise the whiteboards; I'm excited to see how it turns out.

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## Day 114: Mirror Diagrams

We started today with the six cases of curved mirrors—five for concave mirrors and one for convex mirrors. We learned about the principal rays of light to find where the image will form. Of course, any time I draw mirror diagrams, I have to pull out the concave and convex mirrors. They make weird images!

For the first time of my teaching career, I did a geometric proof of the mirror equation. It looked familiar, but I don't think I saw it since the 1990s. Why did I never do it before? It's so cool. The students who loved geometry loved that proof.

As you can tell, it was a lot of me in the front of the room. I felt it. It was needed and useful, but I remembered why I don't like lecturing.

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## Day 113: Reflection & Mirrors

How long does a plane mirror need to be so you can see your whole body? Where is your image located? We experimented with that today. I realize I should by more long plane mirrors so more people are willing to try it out.

We also looked, in detail, about how light would bounce off a curved mirror. There seemed to be a focal point, where the parallel rays of light would all converge. But what about objects that are not infinitely far away? It was off to lab to collect some data.

We got great data, but then finding a function to model the object distance and the image distance was difficult. I need to find a better way than just guess-and-check until I have to show them the quite-complicated fit.

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## Day 112: Pinholes and Reflection

How do you know light travels in a line? We looked through camera obscuras and drew pictures to understand how the light makes the images we saw. Then we practiced a bit before we moved on to our first test of the ray model of light—reflection. We learned the three-pin method for finding a ray of light that goes through three pins, hitting the mirror at the second pin. We came up with a very simple law for reflection.

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## Day 111: Finishing Standing Waves

We finished standing waves today, for the most part. I had students gone to a concert, and I knew some of the pictures would be difficult to draw, so we did a lot of working in groups today. We also explained the results to our sound lab, and our results came out pretty darn close to what we expected.

When I went around the room today, trying to get a sense of where we were as a class, some students really liked this unit. Students told me that they liked how it applied to music, liked how it seemed to make sense, and liked that they could be successful with it. A few students didn't like the unit, but that came from the fact that it didn't feel like the rest of physics. Well, physics contains lots of different models. You're just going to like the beauty of some models more than others.

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