Primary Colors
The misconception I’m going to talk about today is the primary colors. There are actually
2 sets of primary colors, and neither of them are red, yellow, and blue like we were taught
in elementary school. Turns out the reason that there are two sets of primary colors is that
one set is for light (which are the additive colors) and the other set is for pigments
(subtractive colors). The additive colors which are used for light are used in televisions,
the video ipod that I’m holding, video games, multimedia projectors, and lasers. If you’ve
ever plugged in the cables to a multimedia projector you may have noticed that you plug
them into the RGB port. The old projector type big screen TVs had 3 colored lights in the
front: red, green, and blue. Red, green, and blue are the additive primary colors, and
again, they are used for light. The reason that they are called “additive” is that if you mix
red, green, and blue, you get white light. This is easy to demonstrate in several ways.
First, you could take a magnifying glass up to a computer monitor or a television and you
can actually see that the entire screen is made up of red, green, and blue dots. By varying
the brightness of each one of the dots you can make the entire spectrum of colors. Some
things that are counterintuitive about this model of primary colors is that students will be
unable to predict what color you will get when you mix 2 of these primaries. An excellent
way for them to discover the mixing of the additive primaries is to put different colors on
the screen and have them look at the pixels and see which of the red, green, and blue
pixels actually made that color. For example, students would never predict that red and
green will make yellow. You can easily see that if you put a yellow square on the screen
and look at it with a magnifying glass and they’ll see that the red and the green pixels are
lit up and the blue pixel is completely dark. If red is alone you get red, green you get
green, and blue you get blue. Red and blue will make magenta, and blue and green will
make cyan. So the 3 combinations of colors you can get out of those primaries are cyan,
magenta, and yellow. Those happen to be the other set of primary colors: the subtractive
primary colors. Subtractive primary colors are used in paints, pigments, and inks. When
you look at the Sunday comics you can see large pixels that are cyan, magenta, and
yellow. If you’ve ever refilled the ink cartridge in your color printer you’ll know that
you’ve refilled cyan, magenta, yellow, and black. They include black because most of our
printing is black, do a separate cartridge of black is needed. Also, it’s difficult to overlay
cyan, magenta, and yellow perfectly to get black, I’ve already talked about this indirectly,
but the subtractive colors; cyan, magenta, and yellow mixed on top of each other
produces black. They absorb all of the colors of the spectrum and black is the result. This
can be demonstrated several ways. If you look at the cover of a magazine under a
powerful magnifying glass (how high quality the picture is will determine the power of
the magnifier that you will need in order to see this) you will see that the entire picture is
made of cyan, magenta, and yellow dots, much like the pixels on the computer or
television screen. With color newspapers it’s easier to see because they use photos with
less resolution, so you can see the dots far more easily. Another way to do it is through a
digital camera. A lot of the digital cameras now can connect directly to a color printer,
which will actually pull the sheet of paper through 4 times. It will pull it through once
and print the cyan, then when you put the paper back in it pulls through again and prints
the magenta. The process then repeats for yellow and black and it is really powerful for
students to see that process. Both of these models can be shown using photoshop.
Photoshop allows you to go to the color chooser and select which color option you want
to use: cyan, magenta, yellow, and black (called “CYMK”) or red, green, blue (“RGB”).
You can type in numbers from 0 to 100% to change the strength of each one of the
colors, so you can do 50 % red, 75% green, and 100% blue and see the color result. You
can do the same thing with cyan, magenta, yellow, and black. Put 100% cyan, 100%
magenta, and 100% yellow and you’ll get black as the color as you probably expected.
There are lots of ways for students to discover the sets of primary colors. They will be
surprised, maybe even shocked when they hear it, so often in my physics classes I would
have the students do some research on this before I did color mixing demonstrations in
class. Some demonstrations you can use are color filters that you can mix cyan, magenta,
and yellow color filters to get all the colors. There are colored lights, keychain lights, and
spotlights that you can shine on the wall and show the mixing of colors. You can even get
red, green, and blue light sticks and spin the light sticks on the end of a screwdriver or a
drill in order to get a combination of the colors to come out nearly white. The eye is more
sensitive to some colors than others and some colors are more pure and bright than
others. You can use electrical tape to cover up or mask some of the colors and then you
can start to get to mixing when they spin. It’s a powerful demonstration. Most of the large
vendors of science supplies such as Flinn, Science Kit, Arbor Scientific, and Educational
Innovations sell demonstrations for the mixing of colors. The computer is also a powerful
way to model the mixing of colors. There are several color mixing applets on the internet.
Pretty soon we are going to be starting a series of videocasts where we will be doing
demonstrations online and explaining the science behind them. One of those
demonstrations will be color mixing using colored lights.