# Day 23: Wait, Which graph? and Ray Diagrams for Lenses

Today was a full-blown white boarding session.  A number of years ago, I slightly modified a Mind on Physics (MOP)Activity that provided a position graph with the motion of 5 objects depicted on it and a velocity graph with 5 different objects depicted.  Here is a picture of said graphs:

I like the set of questions that come along with it because most promote discussion amongst the students.  Here is my favorite:

Assuming object G starts at the zero position, at what time is it farthest from the origin, t3, t4, or t5?  It really makes the students think.  Today two different groups called drew the matching position graphs to help explain it.  It is the first glance at a changing velocity even though we do not mention the ‘a’ word.

Tomorrow we will throw some numbers at the constant velocity model.

General Physics:

Today was not the most exciting day, especially after seeing the aerial image yesterday.  We learned how to draw ray diagrams to explain (or predict) the images that form in lenses with the object at different object distances.  Pretty standard assignment I’m guessing.  To check the diagrams we ill use an optics bench to physically set-up the situation and allow the students to really see the image and discuss the characteristics.  There is more, but I’ll save it until tomorrow.