Day 21

So I got permission to go to the Wisconsin Association of Physics Teachers Conference next month in Milwaukee. Pretty excited about that, since Destin from Smarter Every Day is going to be a keynote speaker. Since I’m so excited, I decided to post my all-time favorite Smarter Every Day episode.

The physics of cats!

Physical Science: Usually nothing much to say about a quiz day, but it went really well. I changed up this middle portion of our skills unit, and it seems to be for the better. Scores averaged in the B+ category!

Anyone who has ever worked through the Models of Physical Science materials knows about the underpinnings unit. When measurements get introduced you start off teaching the kids why using nonsensical things doesn’t work, and the need for standards of measurement. The pedagogy then moves to teach kids how to use the same paper clip, pen, pencil, and tongue depressor to get comparable data when measuring with these items. Then you introduce unit conversions and ratio reasoning with units that don’t make sense. The idea is to focus on the skill, and not let the students get distracted by centimeters, meters, etc. which they think they understand.

I get it and agree with its ends, but it takes so much time, only to go back and do it all again with SI units. So I skipped it this year. What is the worst that could happen? Well as it turns out, it was much to my benefit to have changed it. Does this mean that going from the initial activity and following the path I did is WAY better? No. Only time will really tell if this is new path I carved was a better method.

Chemistry: They finished their lab practical today. It wasn’t overly pretty. The smallest percent error was -45%. The largest was 700%. I guess we’ll have lots to talk about tomorrow.

Physics of Light: So now that the quizzes are done, we had to find something new to look into. At the start of class I brought up how we had sort of looked at the window in my classroom door. Light went through it (we can see stuff on the other side) and light bounces off of it (we can see us on it form the inside of the room.) So maybe we should look more into that.

I set up my 10-gallon aquarium with tap water and a little coffee creamer. You have maybe seen this setup in the past. Only this time, I did something completely different.


I put the tank on top of some books (who says I don’t use physics textbooks!). The bottom of the tank is glass, so I thought maybe we could see what happens on the way into the water and out.

So back to class. I told the kids that water is just like the glass: light can go into it and also seems to bounce off the surface (who hasn’t seen their reflection on the water surface?) This is better than glass though because we can determine its thickness and also make it so we can see what the light does when its in the medium (just add creamer.)

So then we got busy!

We shone a laser pen into the water, out of the water, every which way. Here are a few of the pics:

Like I said, we did all sorts of stuff and there was lots of “more/less bendy up/down.” So much to the point where I said, “Okay enough. We have to get a little more technical so we don’t get all confused here.” So we did some diagrams and talked about how the light bends compared to if it just kept going in a straight line. We also agreed that the bouncing of particles was just like we had seen before, angle of incidence equal to the angle of reflection.

Then someone said, “What if there was a layer of oil sitting on top of the water? What would that do?”

So I sent a kid to get a bottle of vegetable oil from my storeroom, and we set this up:


I asked if they thought if the light would still bend, and everyone said it should. Well this is what we got:


Notice that it does bend down (compared to if it kept going in a straight line) when the light goes into the oil. The kids did note that the light does bend again when it gets to the water, but nobody really noticed (or at least said aloud) that it doesn’t “bendy down” again. If you look close, it actually is “bendy up” a little compared to if the light particles had kept going in s straight line from oil to water.

So I gave them the option to either investigate the bouncy action (reflection) at the surface boundary OR to investigate the bendy action that occurs when light enters a new medium.

Unanimous: Bendy action!

So we setup an investigation: The Bending Light Lab. Everyone said we could compare the bending by measuring some angles on both sides, and we had some creative ideas. I suggested that in order to keep from dealing with a wavy surface, we should measure from a “normal line” 90 degrees to the surface over to the light. This is because at the infinitesimally small point the light strikes the surface, it would be flat enough to get this normal angle, and thus we wouldn’t have to worry about the wavy action of any substance we might test.

Oh, and they agreed that we should try this for some different substances, especially since the oil and water gave different results. Pretty good idea in my opinion.

Confession: I staged most of these pictures later in the day. I was to engrossed with the discussion to stop for pictures. So thanks to my freshmen helper Josh for once again helping me get these shots for the blog. He doesn’t seem to mind as the stuff we’re doing is “pretty cool.”


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