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The cool idea about light that I’m going to brag about today

September 12, 2011

In my class, we were talking about what happens when some dust floats across the path of our flashlight.  We had mostly agreed that, while we can’t usually see the path, there must be some path because the light gets to the end of hallway where we see it on the wall. I had them now white-boarding in groups, trying to show what the light might be doing when there’s not dust in the path and what happens differently when there is a piece of dust in the path.

One group, in particular, said that the light was both absorbed and reflected when the path of light got the dust particle. I asked them to say more about what they meant by absorb and reflect, and here’s what came up:

First they elaborated on the absorb idea by saying that the dust particle soaks up the light. Later, I asked them if they meant like a sponge–like how a sponge can soak up water and then it’d be wet. By their idea, the dust particle soaks up light and by virtue of having the light, the dust particle is lit. This idea makes sense to me. Without light, things are not visible. With light, they are visible. Their idea very much connects with the light is a substance metaphor. The word soak is, in fact, very similar to words students had used before, like spill and leak when talking about how light got from the hallway to the room.

The group had a much harder time talking about what they mean by reflection (and later decided to drop the word in favor of just absorb). They mostly just kept repeating the word reflecting, so I suggested that it might be easier for them to draw what they meant then to say it. They ended up drawing some lines around the dust particle going every which way, using a different colored marker than the color they had used to show the light coming from the flashlight. I asked them if those lines that had just drawn were light. They were very adamant that these lines were not light and pretty darne certain that light did not go out from the dust particle. I think they mean those lines to indicate that the object was lit or visible. Their reasoning was also very sensible to me. They argued to me that when we look directly at a lightbulb, it is bright. They seemed to be saying, “It is that brightness that lets you know it is a source of light”. Later in a whole class discussion, their argument was more precise: The dust particle was not glowing–not in the same way a the bulb does–and thus light couldn’t be going out in all directions from it. They argued that you simply see the dust because it has absorbed light; but that the dust itself was not a source of light.

At this point, the idea that objects “soak in” light to become visible is completely sensible. In fact, it is a giant step in the right direction. Previously, our thinking had been that you can see light going by you in the shape of a beam. We’ve found since then that you can’t see the beam of light, at least in most circumstances. This group’s idea explains a lot of what we have observed and come to understand: It’s consistent with the notion that we can’t see light going by and that we can see objects. It is also consistent with the idea that you can only see objects if light shines on them.  The same group even earlier had the idea that when you look directly at a flashlight that you aren’t actually seeing light, but that you are  seeing the glass (an object) full of light. They are fairly committed to the idea that we can’t see light, and they are trying hard to tell coherent story of only being able to see objects. So much so, that they don’t want to say you see light when you look at the flashlight, but that you see glowing glass. This is progress along several dimension: thinking that is increasingly accountable to evidence and thinking that increasingly internally consistent.

On Wednesday, I think we’ll spend some time doing observations and experiments to see if we can tell whether any light comes off objects that are lit from a flashlight. Based on this, we’ll have some more evidence to ponder and some more thinking to do.

We’ll also be trying to come to some class consensus about how to we’d like to draw and represent paths taken by light. Right now every group’s diagrams are so idiosyncratic that’s it’s difficult to disagree with anyone. There’s definitely a movement in my class of “We’re all saying the same thing,” and “We all agree with each other.”  I keep having to convince them that we can’t be all saying the same thing if we are all predicting we’ll see different things. This was also true today, I had to make the case that “absorbing” light like a sponge was a different idea that light “bouncing” like a tennis ball. This subject about the tendency to want to politely agree, I suppose, is for another post.

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