here. The one I needed was the 10th grade English version of Physical science, 10EM_P. At right is the index with suggested pacing. I needed to develop activities for chapter 3, Reflection of light by different surfaces. My initial thought was to find some inexpensive curved mirrors. One good choice was a 7.5 cm diameter mirror offered by Teacher Source that is coated on both sides so it is both concave and convex. Unfortunately, even its reasonable $7.95 price was probably too much. I remembered seeing a curved mirror at a NCNAAPT meeting show and tell session made with silver mylar. A rigid shallow cylinder is closed at one end and covered with mylar at the other. Air is pumped out, resulting in a nearly parabolic shape to the mylar. I decided to try my hand at making my own curved mirrors.
I did some searching online and found several DIY videos on making concave mirrors with silver space blankets. This one was helpful but too complex. When I found this one that uses a trash can lid, I realized this could work. The problem would be finding an easy way to change the pressure inside. While sipping a glass of wine at home I had an inspiration. I could mount a Vacuvin wine saver to the trash can lid and pump the air out. The stoppers are only about $1, one $8 pump can be used for multiple mirrors, and this is a LARGE concave mirror. I ordered some strapping tape, silver space blankets, and silicone sealant. I went to the hardware store for some trash can lids. It turns out they have a lot of extra ones because people replace dented metal cans but leave the lid behind. They gave me 2 lids. I followed the DIY video directions as close as possible. The only modification was a larger hole to accommodate the Vacuvin stopper. My first attempt had a few leaks but those were easily covered with more tape. I pumped out the air until I heard the lid start to deform and turned it around. I had a pretty good concave mirror with a focal length a little less than 1 meter. Above is my TA using it to cook some teriyaki chicken. Below is a picture of me using it to reflect and focus the infrared radiation coming from a space heater.
This one also can be used in reverse.
I attached a flexible tube to the coffee can and gave this a try. It worked great. It was very illuminating to see how the image changes as you go from a flat mirror to a convex to a concave. Here is how it looks:
This seemed to be a good solution for an inexpensive large mirror that can be used in the classroom for curved mirror activities. Since the mirror can be created by just blowing in the tube, the most expensive component was the tape. Large syringes also work but you need to kink the tube between multiple pumps with the syringe.
I presented the mirror project at the March PTSOS workshop. I later heard from one of the teachers that had their students make and use trash can mirrors. He apparently was able to manage the safety issues that arise when you bring these out into the sunshine. I am still hoping to make s'mores with my students this year. The space blanket mirrors also drew interest when I used them for my show and tell at the Spring NCNAAPT meeting.
I have more ideas for space blanket mirrors. Instead of a coffee can I will make a cylinder/piston device. A ring would hold the space blanket and seal it at one end without needing tape. A piston and gasket would seal the other end. You could move the piston up and down to create concave and convex mirrors of different focal length. This would make it easy to replace the space blanket material that does wear out after a lot of use. Looks like I need to visit TAP Plastics. This summer I am helping with The Lowell Observatory Eclipse Experience at Madras High School in Oregon. In addition to bringing my spacetime simulator, I am going to build a large convex mirror using a 4' wide kiddie pool. It should give a horizon-to-horizon view of the sky during the festivities and the eclipse. You should come by to check it out!