Adding colors
Look at your computer screen with a strong magnifying lens (there's one in your kit -- use the small lens on the handle). You will see that the picture is made up of tiny colored dots or stripes, of just the three colors red, green, and blue.
Observe how the dots combine to make the colors in the different bands.

This effect is called color addition: we are combining lights of only a few different colors to make our mind think it is seeing all colors. It is a very different kind of combining than was involved in the "color subtraction" activity. So if someone asks, "What is red plus green?", the answer depends on whether it is colors of light (like the little colored dots adding up to make the pictures above) or colors of filters that are being stacked together and put in front of a light or a colored object.


Color Addition Color Subtraction
Mixing red light, blue light, and green light
-- pretend these are colored spotlights		 Combining an aqua filter, a yellow filter,
and a magenta filter
--pretend we are viewing white light
through overlapped filters.
Red light + Blue light=Magenta Magenta filter + Yellow filter=Red
Red light + Green light=Yellow Yellow filter +Aqua filter=Green
Blue light + Green light = Aqua (also called Cyan) Aqua filter+Magenta filter=Blue

These pictures are not actual photographs of real filters, of course; they are made by turning on and off the red, blue, and green dots on the computer screen. You can try to reproduce them with the filters in the kit. To do color addition, you have to shine white light through the filters (to make colored light) and then overlap the colored light beams on a screen. You would need three light stations (or three overhead projectors); it works pretty well and is actually fun to do (it is one of the things proposed in the discussion section).

Overlapping the filters is much easier, but what you will get does not look quite like what the theory predicts. The problem is that the real filters are not perfect -- the blue filter lets some red and green light leak through, and the green filter lets a lot of blue and some red leak through. So a yellow object, seen through the real blue filter, does not look black -- it looks dull yellow.

So our eyes are not telling us the truth, our brain is covering up for our eyes, and the color filters are not quite what they claim to be. It must have been really difficult to unravel this subject!

Box:  

About color vision