Meet Me In The Back of the Room; Bring Nothing

I walked through the ways to draw ray diagrams with lenses with my classes. The last few years, I just told my student, "eh, it's basically the same thing. Let me show you how to draw the three principal rays, okay, do the rest on your own" and it's kinda been a disaster. And when I went through this, I asked both classes if they were bored. They weren't! I guess it's worth the time in class.

Then we had a sunny day and burnt paper with mirrors and lenses. It was great to watch students figure out how much more difficult it is to burn paper with mirrors. I especially liked it when the kids who had the convex mirrors figure out why it was so hard to burn paper; I mean, the convex lenses worked, right?

I just threw the students into the lenses lab. I didn't tell them where the screen should go, so it was great to watch students try to figure that out. They were sure it would act like a mirror, and the real image would be on the same side as the object, but once they thought about the difference between reflection and refraction, they weren't so sure. They instinctually went for large F's; I had to cajole them into making small F's. The data came out exactly like it did for concave mirrors, so it was easy to assimilate the new information.

Even without directions, I was impressed most by the wows! I got from the class. Real images are amazing to high school students, even if I find I a bit bored in my expertise. I get joy from their wows. They were, like me, unsurprised that the equations were the same, which I found heartening. I guess they think like physicists, thinking they can use the same model again and again?

I then took eyeglasses and put them in the light of the projector and saw the diverging from (most of) the students' glasses and converging rays from the old person glasses I bought from Target.

Today, we whiteboarded problems on curved mirror and refraction. And, in both classes, the whiteboarding went quick. And, in both classes, the sticky points were the same—what if we cover half the mirror? why do we have to be careful from where we measure the angle of refraction? How many significant digits should we use? Then we talked more about the law of refraction and when it doesn't give an answer and watched my favorite total internal reflection video.

The class today made me think when is whiteboarding useful. If I know what the questions are going to be, and they are this predictable, does that mean I shouldn't whiteboard? Does that mean my questions are too easy? Does it mean that I should whiteboard and let students come to their own answers through these typical pitfalls?

I told them how to go from the interesting straight line we got when graphing sine of one angle versus sine of the other angle when light enters a new medium. I then told them about the index of refraction and gave them some examples. Then we worked on some practice. 

I have no photos today. Today was hectic. I got two new students, one in each section of my physics first class. I had no notice. That, on top of a lesson that really didn't work for those students because it was too complicated, really made today a hard today. Did I mention it was open house yesterday? I am so tired.

We took a few notes today. I just told them about the difference between concave and convex mirrors and what that means about the sign convention we use in the lens equation. It's kinda boring, and not very hands-on of me, but this move saves me some time. Which I used to get us started to talk about the next concept. We listed the three ways light can behave when it hits a boundary: it can be absorbed, it can be reflected, and it can be transmitted. We know the ray changes direction when it is reflected; let's see if the path of the ray changes when it is transmitted!

When I showed the class the three pins method with the semicircular Petri dish, drawing in the normal and showing the angles really helped because even though the students didn't realize internalize it, it caused students to realize their angles seemed off and then think about how we measure angles. I really liked this new move.