My students got pretty decent results during this activity. I did have several students that questioned if 47 degrees really was the same as 45 degrees. We talked through the set up and the procedure which seemed to convince them. Then I had them put the mirror on top of the protractor and shine the laser at the point where the normal line touch the mirror. They did this for different angles using the numbers on the protractor which they immediately saw that the angles were the same.
Becky, thanks for another good post.
I'm not sure from your description whether this applies, but glass mirrors have a confounding effect: the light actually reflects of the reflective backing behind the glass. The surface of the mirror is glass, and the light actually enters the glass, reflects of the silvery coating on the back, and then leaves the glass. That extra distance (and perhaps the refraction?) makes the lines seem to connect behind the mirror. It should definitely be symmetrical, though.
