# Day 16: The cvpm is finalized and Modeling the Pinhole Camera

Today was the teacher talk part of the dune buggy lab.  Teacher talk may not be the right (or pretty) phrase… but it is, unfortunately, a a good descriptor.  I did too much talking , well actually I asked a whole bunch of questions as we summarized the dune buggy lab.  When discussing the position-time graph, I find some of the advanced kids just want to jump right to the ‘speed is the slope’, and when I say, well, not quite… these student switch to velocity.  I try my best to build the concept first…the slope of this graph is:

1.  the change in position  (later defined as displacement) in a time interval of 1.0s.  We develop this right from the units on the slope,

2. the rate of displacement

OK, lets give this concept it’s own name —> Velocity.  I wholeheartedly buy into the concept first, name second idea (thank you again Arons)

In the end, we wrote the general mathematical representation:  xf=(v)delta t + xi

We also built a new way to picture motion, the motion map.  We built it from a strobe photo of a dune muggy in motion.  Well, to be honest, it was  a series of pictures of a dune buggy on my Smart board.  It would be really awesome if there was an app that would do this for me… I wonder….

General Physics:

Today was about building the ray model from the pinhole camera activity.  We started by agreeing on the observations we made with the pinhole camera.  We then developed the need for a ‘line of light particles’ because we could not draw enough of them and they were so close together, it looked like a line– this became our ray.  I used  bamboo skewers to represent the rays coming off the light bulb and an index card with a hole punched in it as the pinhole.  We were able to physically model everything we observed during the activity.

SIDEBAR: Should have snapped a picture of this….

We also discussed that the light travels really fast — at light speed! yes, that really was a student answer… well, I did ask the question.

From here we talked about what we what we could do to alter the path of the light.  A mirror was suggested… true, but not what I was looking for. I was looking for change the medium.  Upon reflection, I think I should have tried asking  “What could be do to slow down or speed up the light”.  I think this would have been better… more natural to say “change what it travels in”.

Anyway, we built the concept of refraction using the old water tank demonstration.  I defined refraction as the change in the path of light as it travels from one medium into another.  The shift in the path caused by  a change in the speed of light.    Yep, next time, I will ask about changing the speed.

This demo also allowed me to define the normal, the angle of incidence, and the angle of refraction, setting us up for a Snells’s Law experiment tomorrow.