Make ’em predict it… REALLY predict it.

by Stephanie Chasteen on July 20, 2008


This will be the last post from the (incredibly thought provoking) session on Piaget. This is about the value of having your students predict what will happen in an experiment or demonstration in order to have them change their ideas about the world.

For example, there’s a wonderful little demonstration/experiment about optics that goes really counter to our expectations. Take a lightbulb — a clear one so you can see the filament — and a lens, and something to use as a screen (like a piece of paper). Put the lens between the lightbulb and the screen. Use the lens to focus the image of the filament on the screen. Now, predict what will happen if you cover the top half of the lens with a piece of cardboard. What will you see on the screen?

Most students will say that the top half of the image will go away. But, here’s the rub, they have to be really invested in their answer in order to learn something. Otherwise there’s this tendency for them to say, oh, OK, so I was wrong. Just another one of those cases where I was dumb and science tricked me. But… you also don’t want them SO invested in their answer that when they’re wrong, they take it as a personal affront and feel that they have to defend their logic or try to explain away what they see while keeping their original model.

I did this activity with the students that I blogged about two entries ago, and they didn’t express surprise. It was done as a quick activity, in a one-time interaction with these students, in a situation where they were doing this as an extra out of school activity. It was hard to get them to really engage in the prediction. What needed to happen was for them to really articulate what they expected, and why, and argue about it with their peers. Dykstra has used this technique with their students, and he says you can hear their surprise in the room when you cover half the lens, and instead, the entire image of the filament remains (but it gets dimmer). There is an audible gasp.

After discussing this further, his students realized that each point on the lens gets light from every point on the filament. You can also show them that if you cover the lens with a piece of cardboard with a hole cut in it, you can move the hole around and the (dim) image doesn’t move. How can that be? Only if every ray from the lens has the whole filament on it.

So, you really have to own your explanation in order to get the disequilibration necessary to change your model. You, as a teacher, have to spend time on the prediction and on the explanations in order to get that personal committment. Dykstra has them first write down what they think (which gives them some time on their own to decide their personal view) and then share with the group and write down what the group thinks will happen (to give them a chance to discuss with their peers). Only then does he have them observe the phenomenon. Many labs have students come in and start messing with the equipment first. He says that no, we should mess with the ideas before we mess with the equipment, that’s the object of manipulation. You short circuit the process of learning if you look first to see what happens.

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