Color subtraction—reflected color

They say necessity is the mother of invention and it's very true for teachers. I found myself wanting to do a reflected color (color subtraction) lab in my Conceptual Physics class that was observation based and introductory before a lecture. An internet search found nothing I could use so I had to come up with something myself.

NOTE: Any reflected color demonstrations or labs have to be in utter darkness. Any light from an outside source can be reflected off your object and will not produce the results you want.

At first I wrote one that used my light ray boxes and shone light through filters on a paper that students would color with Mr. Sketch markers. When I tried the experiment myself though, I wasn't getting the results I wanted using the filters. I turned to my trusty Inova Microlights in red, green and blue and found that they worked well when shone on the same paper but I didn't have enough for lab groups nor the time to get to an electronics warehouse that carried them locally. It was the night before I wanted to do the lab and I felt like a first year teacher not knowing what I would do the next day.

I thought if I didn't have a light source in the colors I wanted then perhaps I could make them using my computer. I started making images in the colors I wanted and trying to project them before I thought, "Someone must have done this already." Sure enough I found an app called Color Light Changer (free version) which did exactly what I needed it to. The program allows you to choose colors using either HSV, RGB or HEX color systems. To produce as colors with the right amount of red, green and blue as possible and for easy student manipulation I chose RGB. I set it up to produce red, yellow, green, cyan, blue, and magenta with little transition time. [See also RGB Colors and RGB Explorer.]

Now I had a reliable light source in the colors I wanted but I had to still find somethings that were the right colors to reflect that light. Since my kids were in bed as I was contemplating this as I put away their toys and found myself looking at exactly what I needed. First I used a set of four wooden balls that were red, green, yellow and blue. With the help of my patient husband that just wanted to go to bed I took this video of the four wooden balls under the changing light.

I only had one set of the wooden balls which wouldn't work for eight lab groups and I really wanted my students to try it themselves. I was pleased with the results but found that I wanted the other two primary pigments, magenta and cyan. I found one or two Duplos pieces in magenta but since we purposely don't have many "girl" Duplos I didn't think I'd have enough. But after dumping my kids' entire Duplos collection out onto the floor I managed to find enough magenta and a surprise store of cyan! Although humorously heterogenous, each primary pigment and secondary pigment was represented for each of my lab groups.

At this point I was promising we would be able to go really soon ... just as soon as I made a second video using the Duplos pieces:

For the actual lab the next morning students either used their own phones after downloading the app or used one of a few Samsung tablets that I borrowed from another classroom and preloaded with the app. Each group was able to observe the reflected color from the Duplos and of course anything else they could get in front of the light. It ended up being simple to do, relatable and more importantly for any light experiment it worked like it was supposed to.