Color Part I
Name_______________________
Color is one of the most important pieces of information scientists have used for all time. In space it
is one of only two pieces of information we can collect without sending probes out (for millions of
dollars). Color is how we determined that the Sun is a typical star and what the Moon was made of.
It's how we know where the Earth came from and how long the Sun has to live. It tells us more about
the universe than anything we could gather even if we could send out hundreds of spacecraft.
(The two items we can determine: a. How bright is the light is. b. What color the light is.)
First, what is color? Color is caused by different frequencies of light. (Huh? Frequency? Frequency is how often. For instance the frequency of you doing your homework is daily. Wait,
make that the frequency of you doing your homework should be daily. The frequency of you eating
is about 8 hours. The frequency of you changing classes is every 48 minutes.) When we talk about
light frequency is the measure of how often the light wave (sort of like an ocean wave) goes up and
down.
For visible light that frequency is in around 1,000,000,000,000,000 waves every second! Very slight
changes in that frequency are detected in the cones of our eyes to be different colors. Unfortunately
our eyes are very poor detectors of electromagnetic waves such as light. Your eyes detect a very
small portion of those waves. There are many more waves that we can't detect with our eyes. (See
electromagnetic spectrum of your text.) We've defined many colors by the frequency of the light that
we receive. We could do calculations on these, but it isn't really necessary for us to understand color
or how to use it.
Instead we will look into how we perceive light and how light works. The first thing we have to do is
revise the color wheel you learned in grade school. It worked for what you were working with, but it’s
now time to get a little more sophisticated. The primary colors of light are red, green and blue.
Eeek! Heresy you say? Think about it. What are the three colors on a TV screen or computer
monitor? Yep, red, green, and blue. What is the color scheme in a camcorder or electronic camera?
Yep, RGB. How about the lights on the stage? The strip lights the theater uses have red, green and
blue lights in them. So what happened to the red, blue and yellow of grade school? Those are
pigments and they work a little differently. We'll discuss them later.
For light, the primary colors are red (R), green (G), and blue (B). The secondary colors are
combinations of equal amounts of the primary light colors. When you combine R and B you get a
purplish-red color called magenta (M). When you combine G and B you get greenish blue color
called cyan (C). When you combine R and G you get a reddish green color called yellow (Y). Yes,
yellow! It doesn't make much sense from your paint mixing days, but it does for your computer
monitor days. You can combine all three colors to get white light, also.
Your eyes have three different (sometimes four) different types of color detectors. You guessed it,
RGB. You have cones (the things in your retina that detect color) and most people have three
different types. Some people have two different types of red, so they are able to detect red in two
different peaks. When you combine detection by two different cones, you get the other colors. The
rest of the colors you see on your TV or on your computer are made up of those three colors, or lack
thereof. Check out the color settings on your computer. Most have the three colors and you can vary
the intensity of each to get whatever color you need.
The definition of complementary colors is different for a physicist than they are for an artist. (But the
colors are similar and you find them the same way on a color wheel.) For a physicist, complementary
colors are two colors that when combined in equal intensities make white light. R and C are
complementary. B and Y, G and M are also complementary. You can find them because they are on
opposite sides of the color wheel. (Just like in art class!) They are important to physicists because
they help define what light we are missing and what is there. Complementary colors are important to
artists because they are two contrasting colors that go together. Classic school colors are often
complementary colors.
When adding colors of light together people often refer to it as color by addition. When you put more
colors together you get more light. When you add pigments you get color by subtraction. More on
that later.
Today we are going to do a lab to help you remember the color wheel for light.
Light (color by addition)
Name________________
Fill in the seven sectors shown. Use the three primary light colors in the large outside circles. (RGB
system) (Remember, paints and crayons aren’t ideal pigments.)
Y=Yellow C=Cyan G=Green R=Red B=Blue M=Magenta W=White K=Black
Each circle represents a spotlight of the color indicated. Fill in the missing light colors.
Color Part II (Pigments)
Name_______________________
Now we get into the second half of the color analysis. This is often referred to as color by subtraction
or pigments. Pigments absorb one or more colors of light and reflect the rest.
Ideal pigments are familiar to anyone who does any work with the four-color printing process, color
copiers, and color inkjet printers. The ideal pigments are ones that absorb one color and reflect the
other two. The ideal pigments are cyan, yellow and magenta. (Dang, those elementary school
teachers were still lying to us!) The abbreviation for this printing process is the CYMK (cyan, yellow,
magenta and black) system. Check the color ink cartridge in your ink jet printer or the toner
cartridges in your color copier. You can get most colors with this process, although you can't get an
unlimited number of colors because you can't raise the intensity of the colors beyond the four inks you
put into the cartridges.
Here's how each color works. To see a color from a pigment or through a filter the pigment has to
reflect the correct color of light (or transmit for a filter). For instance to reflect magenta you need
equal intensities of red and blue light, but no green light. Therefore magenta pigment must absorb
green light and reflect red and blue. Cyan has to absorb red and reflect blue and green. Yellow
has to absorb blue and reflect red and green. (Note each ideal pigment absorbs its
complementary color.)
If you mix pigments, each pigment absorbs some color or colors of light so each pigment keeps
absorbing more and more light. (color by subtraction) If you mix cyan and magenta you get blue.
(Cyan absorbs red light and magenta absorbs green. The only light left to reflect is blue.) Magenta
and yellow makes red (magenta absorbs green and yellow absorbs blue) while yellow and cyan
make green (yellow absorbs blue and cyan absorbs red).
Now you have to make a separate color wheel for pigments. It's the more common one, but it's not
exactly like the one you have in art class, but we'll explain that later. You can still find complementary
colors (two colors that combine to make white light) on opposite sides of the color wheel and they still
go together.
Pigments and filters work the same way, except filters are made to pass light through. Lighting
managers for the stage use these to their advantage all the time. Remember, if a filter doesn't allow a
light to get through, that light can't reflect off the items on the stage. This can be used to dramatic
effect.
I saw a scene once where all the actors were dressed in yellow and they went through the
scene in normal (amber) lighting. At the end of the scene a terrible accident was supposed to
have happened and people died. The director changed instantly from amber to blue lights (a
blue filter over the light allowing only blue light through, absorbing green and red light) - and…
All the actors' costumes turned from bright yellow to black - blue has no red or green light to
reflect off the yellow pigments in the costumes.
What about those Prang watercolors I used to mix and or the acrylics I use in the art department?
Well, they aren't ideal pigments by any stretch of the imagination. In fact the watercolors you used in
grade school were mostly the cheapest pigments possible to slap into a little case. The yellow wasn't
a perfect yellow. It reflected lots of red and green, but it also reflected some orange and some yellow
light (Absorbing blue and violet). The blue reflected lots of green and violet as well as blue.
(Absorbing red, orange and yellow.) When you mixed the blue and yellow watercolors together, all
the colors were absorbed except for green.
Even with more ideal acrylics or oil paints, you have to be very careful of how you mix the colors. If
you mix a very bright red with a very bright yellow you get a brownish orange - not what you
expected. Artists have to do much trial and error to get just the right colors for a painting. They are
truly scientists experimenting, often writing down what worked and what didn't (recording in a notes).
If you want to see how pigments are used, check out the Sunday comics under a converging lens.
You can see that they are printed from multiple dots overlapping to form different colors. This is very
similar to the method they use to print color pictures in a textbook, except the textbook's dots are
much, much smaller. Some of the most valuable Pokeman cards are misprints where the printers
messed up the print and the four colors (CYMK) don't overlap perfectly and the card looks like it is out
of focus. We even had to return some of our textbooks to the printer because of the same problem.
(Strange that misprint Pokeman Cards, Baseball Cards and Postage Stamps are rare and valuable.
Misprint textbooks are worth much less and usually destroyed.)
Spectrum
There are three basic ways to get a spectrum our visible light. (although there are many variations)
First there are prisms. They take advantage of the fact different colors (frequencies) of light travel
at different speeds through a medium. Red changes speed the least, so it bends the least. Violet
changes speed the most and therefore bends the most. This creates some separation when white
light goes through a prism.
Note: this can create a problem for lens telescopes. Since lenses are really a bunch of prisms
stacked on top of each other, the lenses also separate the light into the spectrum. This is
known as chromatic (color) aberration. This is a result of all colors not focusing to the same
point. Look through a hand lens. At the edges you will notice that things change color slightly.
This is the color aberration. Even camera lenses have this problem. (Mirror telescopes don't
have this problem!) High quality telescope and camera lenses actually use two lenses of
different glass (different index of refraction) glued together to adjust for this. It makes these
lenses more expensive, but they work better.
Diffraction gratings also separate the colors into the ROYGBV. You've seen this when looking at
the etched side of a Compact Disc. You see a spectrum because of the waves interacting with each
other. We will be looking into that phenomenon later, but you will be able to use some diffraction
gratings later to look at some elements and see how we use them to study color in even more detail.
Rainbows are my favorite way to make a spectrum. To study them in more detail, first log on the
Internet and check out the Earth and Sky website (our website has links to the shows you need.)
http://original.oprfhs.org/division/science/faculty/kmccarron/ils-1/rainbows.html
Primary rainbow
Secondary rainbow
Once again, you notice that there is some refraction as well as some reflection to make a rainbow.
Rainbows also take advantage of the different indices of refraction for different frequencies of
light. As you look at a rainbow, the Sun must be at your back and the rain drops must be ahead of
you. There's always a double rainbow, most often the second bow is too dim to see.
Homework Questions
1. Do rainbows show reflection or refraction? Explain.
2. What conditions do you need to have a rainbow in the sky?
3. What is the difference between two different colors of light?
4. How do we detect color?
5. An actor stands where two spotlights cross. One is red and the other green. What color light is
the actor standing in? What color are the shadows?
6. Why is the sky blue?
7. Why is the sunset red?
8. What is the absence of all color?
9. A ship painted red, white and green sinks to the bottom of the ocean, where there is only blue and
green light (the red is absorbed above). What colors will the ship appear to divers using the
natural light of the ocean?
10. What are the ideal pigments? Where might you find them?
11. What are the primary colors of light? Where might you find them?
Pigments (color by subtraction) Name________________
Fill in the seven sectors shown. Use the three primary pigment colors in the large outside circles.
(CYMK system) (Remember, paints and crayons aren’t ideal pigments.)
Y=Yellow C=Cyan G=Green R=Red B=Blue M=Magenta W=White K=Black
What is the difference between color by addition and color by subtraction?
Using the diagrams below determine what color light will get through the filters.
Fill in the missing pigments using the CYMK system.