Color Mixing

Background Information

White light is a mixture of all the colors of visible light. You can reveal this by using a prism or by using a spectroscope. Students can create a simple spectroscope using only a CD-R and some packing tape. (Full directions can be found here). Separation of the different colors of light is really easy. It is a bit harder to put them back together again to get the white light back, but it is possible.

There are some really good activities in the Digital Earth Watch Software guide for investigating how red, green and blue can be combined in different amounts to make all the different colors we see. Some of these can also be done with free simulations on the web. However, in my experience students don't really buy that red light plus green light mix to make yellow light when their only experience with this comes from the computer based activities or videos. If you really want buy in you have to show it to them.

You can buy really good lights from science supply catalogs or you can get pretty good ones from a hardware store. I prefer the hardware store approach. The lights are significantly cheaper that way. You can get compact fluorescent lights in a variety of colors, but all you really need are red, blue, and green.  You'll also need a "Shop Light" fixture for each. Total cost for three bulbs and fixtures is under $50. I'm going to outline a few interactive lecture demonstrations that should help students determine how we see perceive the visible spectrum.

Ideally you'll be able to cut off all light in your room. This may mean covering windows to the outside as well as windows in your classroom door. The darker it is, the easier it will be for you to illustrate how our perception of color works. At a fundamental level our eyes are simply sensors that respond to visible light. We have cells in our eyes that are stimulated by specific wavelengths of the electromagnetic spectrum. Our brains interpret that stimulation as color.

Richard Feynman: Seeing Things

Big Idea #1: Objects are the color they reflect

  • Red Laser Pointer (or an infrared thermometer with laser guide)
  • Optional Green Laser Pointer
  • Red, Green, and Blue Lights
  • Objects of various colors (keep hidden till needed)
  • Dark Room
Teacher Setup Notes:
You'll need a laser of some sort. If you don't have a laser pointer you can use one of the infrared thermometers that is included in the spectrum kit. These have a laser guide to show you the exact spot that is being measured.

For the colored objects it is important that you check them out with your lights before you have a room full of students. Nothing is more embarrassing that holding up a blue ball when the red light is on and having it still be obviously blue. Your lights will not be perfect so you really need to check stuff out to make sure it will look the way you expect it to with your students.

Ideally you'll want objects that are red, green, and blue, but you may also want some yellow, orange, cyan and/or purple objects as well. When you hold your red object in front of the blue and green lights it should look black. You might not find the perfect objects, but your students will understand better if you can find ones that are really close. The table below shows the colors should see when our three lights are shined on different colored objects.

 Object color red light green light blue light
 red red black black
 green black green black
 blue black black blue
 yellow red green black
 orange red green black

There are a couple reasons it might not work perfectly. The first is that our lights are not perfect. Our blue light gives out mostly blue, but also has some other colors mixed in, for example. The other big reason has to do with the object itself. A red object reflects red light, but if it also reflects a little blue we might not see that in white light, but it is possible to see it with the blue light. It is also possible that the object is actually fluorescent. We'll talk about fluorescence later.

Do we see light going by?
Simply ask the question. Then propose shining a light on a spot the students can't see. Ask if they'll be able to determine if the light is on of off. This is where we need the red laser pointer. I typically point it at the floor behind my teacher desk. Press or don't press the button and ask, "Is the light on or off?" Students won't be able to tell. While still holding (or not holding) the button simply move your hand into the beam to reveal whether it truly is on or off. 

If you have a green laser pointer you can repeat this demonstration in the dark. When doing this demonstration in the dark you may need to block the tip of the laser pointer from the students view. First repeat the demonstration with the red laser and then ask, "If I use a green laser will anything change?" Be sure to allow students a chance to predict and then show them.

The green laser should be visible. Ask the students, "Why can we see the green laser beam, but not the red laser beam? Check your neighbor." After a few minutes of student discussion ask for their answers.

Are we seeing the light go by? If not, what are we seeing? Most green laser pointers are substantially brighter than red laser pointers (so you need extra caution here). What we're seeing is light bouncing off dust particles in the air. The dust also reflects the red light, it is just dimmer so we don't see it. If students look closely in a dark room the can see the red light reflecting off the dust particles as well. You can enhance the effect by banging a couple chalkboard erasers together above the beam.

Most students have also seen movies where the heroes use some sort of spray to reveal the lasers used for a security system. This is the same idea. We don't see the laser we see the light from it reflecting off small particles in the air.

What color is it?
You will need you red, green, and blue lights. Start with the red light turned on and the others turned off. Ask students why everything looks red. Seems like a simple question, but we want them to see that if our light source is only giving off red light that is the only light that will get to our eyes. 

Hold up your various colored objects one at a time and ask the students to note what color they see when they look at the object and also a prediction for the actual color of the object. This is a combination of the prediction and experience phases of interactive lecture demonstrations. After students have gotten a chance to guess the color of each object turn off the red light and turn on the green light. Again, have students list the color they see for each object and the color they guess for each object. Once done repeat again with the blue light. For this portion of the demonstrations it is important to only have one of the lights on at any given time.

Once you've gone through each light turn on the room lights and show them the objects. Were they right? Were they wrong? Why? Check your neighbor. It's ok if they don't have an answer right now. They're still building knowledge. Ask which colors of objects reflect which colors of light.

This works if you have a whiteboard and markers that work well with your lights. When you write on the board with a red marker and the look at it under the red light the writing disappears (or at least gets very faint). Why? Check your neighbor. Did the writing really disappear? Is the writing still reflecting the light? If so, how come we can't see it?

The whiteboard reflects red light and the red writing reflects red light. We end up having no contrast. Our eyes can't pick up the difference between the writing and the whiteboard. If the writing were not reflecting the red light it would look black on a red-board. 

Big Idea #2: Colors of Light Mix

This demonstration relies on having your red, green, and blue lights set up pointe towards a whiteboard or white screen. Start with the red light on. Ask students to predict, "What color will we see when we add the blue light?" Most will say purple. When you turn the light on they will feel they got it correct when they see the magenta light that results. Then do green plus blue light. Students will guess bluish green and will be rewarded with cyan.

Finally have the green light turned on and ask what color students will see when you add the red light. Most will tell you that red and green make brown. When you turn it on you should get yellow. If we start with green and then add more light then it should get brighter, not dimmer as it would be for brown. Students mostly have experience mixing pigments, which is subtractive color mixing. The more different pigments we mix in the darker it will be. When we mix light we're doing additive color mixing. That means as we add more it gets brighter and brighter.

Students may notice weird shadows during these demonstrations. When you have two of the lights on all the shadows are colored rather than black.

Place an object such that it casts two shadows. If you have the red and green lights on then you will get a red shadow and a green shadow. Ask, “If I turn off the red light which shadow will vanish?” The shadow that looks red will become black and the green shadow will go away.