Day 38 and 39

Day 38 was Thursday and Day 39 is Friday, which is today. Today is in-service, so I’m combining both of those days into this one post. There will be a separate post later after I get back from the Wisconsin Association of Physics Teachers Conference tonight. I’m heading down to Marquette University to see Destin from “Smarter Every Day” speak tonight. Pretty stoked about that! In fact here is a sweet video from Destin that you should watch right now.

Goggle up!

Physical Science: Today we looked at how to classify matter. We looked at states of matter and pure substances v. mixtures. It was basically note taking with discussion of examples tossed in. It’s amazing how kids who are drowning in notes suddenly perk up at the mention of Kool-Aid or chocolate chip cookies (homogeneous v heterogeneous mixtures.) We also talked about how you can have mixtures that involve all combinations of the states of matter.

My favorite to discuss is gas/liquid, because we get to talk about why soda tastes better when you first open it. The mixture of carbon dioxide gas with water makes carbonic acid, which creates the bite.

“Why doesn’t Mountain Dew bite your tongue like Coke?”

“Is Mt. Dew as bubbly as Coke?”

“No…okay, got it!”

I love talking about food in class. Some day I hope to have a food science class, and teach about the importance of the maillard reaction when cooking food. Such a tasty reaction!

Chemistry: Today we continued on our Pressure lab tests. We did amount of gas versus the pressure today. Data was collected quickly, and kids were able to get their post-lab analysis done. Monday, we look at temperature effects.

Physics of Light: Today we started off by looking over our quizzes from last week. The biggest discussion about the quiz centered around the learning objective that asked kids to explain why total internal reflection occurs. The universal explanation centered around Snell’s Law and how when solving for the angle of refraction, you end up taking the inverse-sine of a value greater than 1.

“This is not possible, so TIR occurs.”

Of course my question to them was, “Does light understand algebra? Has it ever heard of Snell’s Law.”

Maybe some teachers would take their argument, but when looking at qualitative explanations, I want to know that they know why stuff happens. Why doesn’t the light escape the the medium. Light particles are not busting out a TI-85 (the greatest calculator ever!) and figure out nope, we aren’t going to make it! So we covered why TIR happens, and moved on.

We then proceeded to look at how to change our model for ray diagrams when we have a diverging lens instead of a converging lens. We know from the water lens demos, that those light rays are NOT going to form a real/aerial image since they never converge at a point. So I suggested that we go and look at what we see for the image before we do anything at all.

This is the image we observed. It is noticeably smaller than the light source, so they knew they weren't just seeing the light source on the other side.

This is the image we observed. It is noticeably smaller than the light source, so they knew they weren’t just seeing the light source on the other side.

So we knew that the ray diagram had to be similar to the one we did for the converging lens when the object was super close. So I modeled for them the three principal rays that you can use for a diverging lens. See the diagram below.


Ray 1) A light ray leaves a point, travels parallel to the optical axis, and the diverges away after passing through the lens. The angle it takes can be traced back to the focal point on the same side of the lens as the object.

Ray 2) A ray leaves the object at an angle that lines up with the focal point on the opposite side of the lens (hence the lightly drawn line on the right side) and diverges such that it ends up traveling parallel to the optical axis.

Rays 1 and 2 would travel to our eyes, but our brains trace them back in a straight line to the other side of the lens (opposite our eyes.) this is why there is a dashed line tracing backwards. This dashed line represent where our brain thinks light traveled, but actually does not.

Ray 3) Travels in a straight line through the center of a lens.

This ray, along with our two dashed lines all meet up at a virtual convergence point. I call it virtual, because at this point, where the image would be forming, there is actually only 1 real light ray. The other two are projections that our brain creates. Still, our brain IS where sight takes place (the eye collects the data), and we only see what our brain can interpret. So there you go.

The rest of the hour was work time on the mega packet I gave them for Monday.

Props: I have to be totally honest here. I was no expert in geometric optics until I took it as a grad class at UW-Oshkosh back in 2008 or 2009. Jeff Elmer, an awesome teacher in Oshkosh taught the class, and I have to give him all the credit in the world for teaching me about this stuff. Not only how to do it, but how to properly craft the pedagogy behind how and why we do things. I still look back at his “teacher notes” on how to do things. Without them, his outlines, his expertise, I would be totally lost.


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