Thursday, April 7, 2011

Time to Bounce

I usually begin reflection with an investigation of the full-length mirror.

This year, we prefaced that with the question "How does reflection work?"  They grouped, I gave them a rubber ball, and they went "to the boards!"

I was mostly looking for the law of reflection, and I got that from many:
  • This group rolled the ball off of a meterstick and traced the initial, final, and bounce points.  They then used a protractor to determine that the pre-bounce angle equals the post-bounce angle.
 Another couple of groups did the same sort of thing, but bounced light off of a mirror, with the protractor vertical, measuring the incident and reflected angles. 

One group set up so that they measured the horizontal distance of the initial, final, and bounce points - doing a slick end-around on the sine function!
  • This group looked at reflection off of a curved surface (a flexible meterstick), and used the ball to simulate it.  They picked up on some good trends, though they communicated more verbally than they left on the board.  We'll get to reflection off of a curved mirror in a few days, but it's great that we're noticing already that it's not really any different than reflection off of a flat mirror (or anything else)!

Almost everyone measures the angle between the "mirror" and the path the first time out, which isn't what we usually do (the angle between the normal and the path is more useful, but that doesn't happen until we get to refraction).  No worries - the concept's there.
  • This group did a very good job comparing the differences between the ball model and the light model: mostly, it's about the effects of gravity on the ball.  Under most circumstances, gravity's influence on light isn't noticeable, but there are notable (and awesome) exceptions, of course.  The first group above also noticed a difference in the speed of the ball after the rebound, which they dissociated from light's behavior.
Evaluating the limitations and implications of your model, on the first day in a new topic?  How much more do you want?!

While the law of reflection was nominally what I was "after," a great deal of other discoveries were made and shared with the class.
  • This group figured out the law of reflection and noticed some things about the transmission and reflection of light by paper.
  •  This group initially thought that the mirror might make light spread out (because the spot of light on the mirror from the flashlight was smaller than the spot on the wall after the reflection), but then realized that it only appeared to do that - a laser beam didn't spread at all.  The reason?  It was right there in the ray diagram the whole time: the flashlight's beam spreads out naturally, and the reflection had caused the path to lengthen, so the mirror hadn't actually done anything!
  • This group came up with the law of reflection (with a snazzy diagram) and also accessed some previous information about color, and why different objects are different colors.  Even better, they were able to use the law of reflection and some ray tracing to show why letters can appear reversed in the mirror.  Several groups noticed it, but they were able to put the pieces together to figure out why.
  • Finally, this group (which has been doing very well with chains of reasoning) took a tree-type approach to organizing their knowledge about light.  It wasn't what I had in mind, but they were getting good work done, so I let them see where it went.  I'm glad that I did: they were able to jump ahead a day and determine the difference between specular (mirror-like) and diffuse reflection!  There are diagrams of each off to the left side.

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