Day 35

So I spent the weekend up north, walking the woods on my father-in-laws land. 122 acres of northern Wisconsin woods. It was 55 degrees, sun shining. I grew up camping, hiking, and fishing, but really exploring nature was not something I focused on. Boy was I missing out, but I am loving trying to make it up.

Physical Science: Guess what we did today? If you guessed standardized testing you would be right.

“How does anyone get any work done with all these meetings?”

Chemistry: We reviewed the assignment from Friday dealing with air pressure. It was a good discussion, because it brings up real world connections. Kids never think about how a water pump works or why its harder for athletes to perform at higher elevations. They also do not thing about gravitational affects on the atmosphere. Since the particles are all flying around, they must be unaffected by gravity! Simply not true, nothing escapes gravity.

So we spend time talking about why water makes a bad choice for a barometer compared to a more dense fluid. We also look at why there is difference in atmosphere at different elevations. We also talk about why when they go to McDonalds they cannot hook together a whole bunch of straws and hope to “suck up” soda to the top of the Playland slide.

Physics of Light: Today was interesting. We finished the bridging activity to get us looking at how we can manipulate light, and what we can then do with it. We started out looking at a water lens (basically made up of a short piece of PVC pipe with watch glasses glued to the edges, then a hole drilled out and capped with a cork). I also have two of these, one where the watch glasses curve outward and one where they curve in.

CIMG1736 CIMG1737

Students predicted that the water lens that curves out, converges the light rays, or makes them all bend together to a point. Then we tried the lens that curves in and they found that those light rays diverge, or bend away from each other.


Converging water lens.


Diverging Lens.

Then we drained the water lenses, re-corked them, and then put them under water in my aquarium. This crates a lens that has a lower index of refraction compared to the surrounding medium. From our flash quiz on Friday, students predicted that the rays would do the opposite as they passed through each lens, and they were correct.


This is the diverging lens, but water-air-water combo.


This is the converging lens, but water-air-water combo.

After suggesting that we investigate these lenses further, and disappointing them that we would not be studying air lenses, we started to look at how to diagram behavior of light through a lens. SO that’s when I gave them this handout. I start off by talking about the parts of the diagram that have special meaning: optical axis, focal point, the object, etc. Then I share with them the rules governing the three principal rays. Once they discover that the three principal rays that leave the “tip” of the object come together at the convergence point, we can draw more light rays from the same point on the object that go to this point.

I then have them practice by adding the rays that leave the bottom of the object. They quickly find that the convergence point for these rays is directly above the first convergence point.

“What is happening at that point?”

“Is there an image there? Or wait, a reproduction? Wait, you said image and reproductions are not the same, so which is it?”

“How do you know that there is an image or reproduction at that spot?”

Silence. Nadda. But then I point out each lab table has the same setup that is on the paper, and that we should go look at it and find out.


“Maybe tomorrow…”

Additional Note: When we start doing the diagrams a student asked why I would draw the light going to the center of the lens, then bending, instead of looking at the refraction at both sides. This gave us a good chance to talk about thin lenses, and models once again.

The student who asked knew that there wouldn’t be one light action, it should be two. I managed to capture this with my water lenses.

Diverging Lens, you can see the spots on the right are where the laser first hits the lens, and how ti does refract as it passes through the lens.

Diverging Lens, you can see the spots on the right are where the laser first hits the lens, and how ti does refract as it passes through the lens.

This is the converging lens. Again you can see the refraction from one side of the lens to the other.

This is the converging lens. Again you can see the refraction from one side of the lens to the other.

So the big deal here is that these lenses are not thin lenses. The path change is very noticeable from one side of the lens to the other. I then talk to the students about how it would be different if the lenses were super, super thin.

“The path length would be so small, it would act like there was only one bend instead of two. That is why we model it that way.”

“Okay, got it.”


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