In my physical science course for future elementary school teachers, I am going to begin on the topic of light, which is where we will stay for about 4-5 weeks, following a lot of the notes and facilitators guides from SGSI.
I am amazed by how much inquiry you can do with a mag-lite. Probably on the second day of class, we are going to do a maglite dissection, which will our starting point for generating questions about how light works. Here is what a mag-lite looks like when you take it apart.
I’ll ask students to examine their maglites, fiddling with any adjustment to see what (if anything) they do. I’ll ask them to take it apart, looking at each the parts carefully, and making a diagram. I’ll ask them to label their diagrams, coming up with a name for each part and a conjecture about what each part is for.
Either during or after the dissection, I’ll ask them to take some notes on things they notice and questions that arise as they do a “maglite potato head”, using different configurations of the parts to see what effect they have on the pattern of light. For example, they might try out some of the following arrangements:
They can also hold the reflector over the shaft by hand and move it around. They can see what effect the cover has, or whatever else they want to try.
Through diagramming, whiteboarding, taking notes, and class discussion, I hope to make contact with some of the following questions:
- What is the adjustor doing?
- Why does the flashlight show a dark spot (in certain configurations)? When does it occur and not occur? Why?
- Why are some areas of the light pattern bright than others?
- Why does the pattern of light seem to show “rings”? Why do rings move when we adjust the reflector? Why do they move the way they do?
- What does the reflector really do? How does it work?
- What is the shape of the reflector? Why is reflector shaped the way this is?
- What’s the best positioning of the reflector? Why?
Hopefully we’ll begin building models and establishing rules for light with some common goals in mind of explaining issues pertaining to the dark spot. Along the way, we’ll have to accomplish a lot, which will eventually necessitate us breaking into different research teams. As we develop various models and rules, there will be new predictions and observations to make progress, refine our ideas, and start to build support for certain models over others. Some of these include:
There are basically three regions of interest with the reflector that students will need to come to distinguish: When the bulb is near the focus of the parabolic reflector; when it’s below the focus, and when it’s above the focus.
Distance to Screen Effects
This is pretty complex, but it’s also interesting. For example, in certain configurations the dark spot in the middle is always present independent of the distance to the screen. In other configurations, the dark spot in the middle is there when far from a screen, but it disappears when you get close. Yet, in other configurations, there is no dark spot no matter where you are.
Effects of blacking out parts of the Cover
In certain configurations, blacking out the left half of the cover (e.g, with a permanent marker) will result in the left half of the light pattern being dark. In other configurations, no light is blacked out. Yet, it other configurations, blocking out the left half makes the right half go dark on the screen. Students can also block either the center of the lens or the edges of the lens, or a line through the center, and other interesting things happen.
Reflected Light vs. Direct Light (and patterns of lighter and darker regions)
Pretty soon, it becomes apparent that some light goes directly from bulb to screen, while other rays bounce off the reflector. Theoretically and experimentally, you work try to distinguish them, and this helps make progress with certain questions and ideas.
It turns out that shadows act VERY different near and far from the light bulb, and they also act very different regions in concert with different configurations of the reflector. For example, in some configurations it’s almost impossible to block out the center with out basically blocking the whole flashlight out. Other configurations lead to an exaggeration of shadows near the center, making the shadows look bloated at the center; or contrary makes shadows shrink at the center. Other effects with shadows arise such that when you poke your finger in front of the flashlight, you produce not only one shadow but two.
Light on Plane and Curved Mirrors:
Of course, we’ll need to develop some rules about what happens when light becomes incident on mirror. For this, some groups will have to investigate this. Some might start by develop rules for plane mirrors. Laser pointers and protractors can come in handy.
Of course, we won’t stay with maglites and darkspots forever. We’re going to want to move forward to pin hole theatres, explaining how those work, and building toward and understanding of blurriness, which may or may not lead us to studying lenses and the eye. Then, of course, we’ll move on to studying color. We’ll build spectroscopes, eventually take photos of spectrum, and use video tracker to analyze them, and start to build rules and models of color. I’m going to have them read some cases about color blindness and after effects. A big puzzle I eventually hope to make contact with is the issue of yellow, yellow, and yellow, and why magenta isn’t in the rainbow.
But right now, I’m pretty obsessed with maglites. Who thought that with a light bulb and a mirror you could investigate so much?
if you get a chance, please shoot me a quick line about how you will build your spectroscopes?
Something like this:
If you can take digital photos, then you can use video tracker to easily plot brightness vs. distance and get graphs for the intensity of light at different frequencies.
Depending on what model maglite you get, the bulb alone is a pretty good approximation of a point source of light. There are some demo/investigations that can be done with point sources that you might try.
Yeah, I agree. That’s the beauty of taking everything off and getting the mag-lite “candle” or “torch”. We’ll certainly have to investigate what the maglite “torch” (or point source) is doing in order to figure out what effect the reflector is having on the light.
Where do you get all the maglites from? They are pretty pricey…