Monday, April 16, 2012

Taking Reflection Models for a Spin

In Physics this month we've been modeling reflection of light and the perception of images.  While we've applied that to both plane and curved mirrors, the principles are the same. A recent assessment, however, showed some confusion about the difference between specular (mirror-like) and diffuse (scattering) reflection - the reason that plywood is a terrible isn't that it's not reflective, but rather that its rough (at the micro-level) surface sends light going in many different directions. 

The other thing that usually takes a bit to really congeal for students is that the only way that we see things is for light to come from them (emission or reflection) and into our eyes.  There's a strong desire to believe that you could see a laser beam going by. Even though students have experience with laser pointers, they segment that experience and instead believe what they see on TV. :)

To attack both of these concepts, I had groups whiteboard some ray diagrams today to predict what they would see when they looked into a mirror on a wooden post.  They were set up at an angle to the mirror, and the lights would be turned off (and the blackout shades closed).  The only light would come from a flashlight that they would hold from their position that was not directly in front of the mirror:

I wanted them to tell me what they'd see in the mirror and what they'd see when they looked at the wood in this scenario.

They went to work whiteboarding, and more or less the only things that I'd say to them were: "does that follow the law of reflection?" and "how is it that you see things?"  The whole process of getting students to see for themselves when something that they've done contradicts a model that they know and agree with is a very difficult one.

Some of their boards:

Scientists and mathematicians really value consistency - every piece of evidence and analysis must tell the same story in order for the pieces to fit together comfortably, but students are extraordinarily willing to let contradictions sit there, frequently not being aware of them because they're not looking for connections and thinking about the implications of what they're seeing or producing.

After a bit, all of the groups had ray diagrams that fit the models, and which told them that we'd see the wood, but not the mirror.  There were just a few holdouts - students that couldn't quite believe that we could look at a mirror with a light shining on it and see just black.  Hopefully having enough time to really commit to their prediction made the evidence to the contrary enough to break the misconception, though that's a tricky process.

The black fabric below served to frame the scene a bit but had another purpose.  We get to contrast the fabric's properties with the wood's - both are rough, but the fabric absorbs a lot of light, while the wood's pretty reflective.  Even better, contrasting the black fabric (which absorbs lots of light, but isn't perfectly absorptive) with the black electrical tape securing it to the pole was another example of specular and diffuse reflection.  The tape also absorbs a lot, but its surface is much smoother than the fabric, so the light that's reflected went over to the other side of the room, meaning that the tape (like the mirror) appeared much darker than the fabric.

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