Facilitator/Educator Guide: Discovering the Colors Behind Afterimages
What happens when you stare at one color for a long time? Why do you see a certain afterimage? Discover why afterimages have certain colors by using a computer, a stopwatch or clock, and colored pencils.
We perceive color using cells in the back of our eyes called cone cells. There are three different types of cone cells, and each roughly responds to red, green, or blue light, the three additive primary colors. For example, when you look at a red image, the red cones are stimulated and tell your brain that the object is red. The different cone cell types work together for you to see other colors, which are mixtures of the three primary colors. For example, if you look at a purple image, which is a mix of red and blue, both the red and blue cones are stimulated. When all three primary colors are mixed, the result is white. All three types of cones are stimulated to see white light.
If you look at one color for very long, the cone cells can become fatigued and temporarily do not respond. For example, if you stare at a red object, and immediately look at a white area afterwards, you will see an afterimage that is the same size and shape, but is a blue-green, or cyan, color. This is because your eyes use the red, green, and blue cone cells to perceive white light, but because the red cone cells are fatigued, you do not see red. You are left seeing with only your green and blue cone cells. After several seconds, your fatigued cones will recover; the afterimage will fade away and white colors will appear normal.
In this science activity, students will view colored afterimages to learn how cone cells in their eyes work together to help perceive different colors.
This science activity can be used as a starting point for a variety of science and human biology discussions. Here are a few examples of questions that can be used to start a discussion:
- How can the colors cyan, yellow, or purple be made by mixing the three primary additive colors? What color is made when all three primary colors are mixed together? How are other colors made?
- What do you expect to see after staring at the color image for 30 seconds? Why?
- Where are the cone cells located in your eye? How are cone cells and rod cells different?
- Now that you have learned how cone cells help you perceive different colors, what do you think causes color blindness?
- Why do you think it takes a longer amount of time for the afterimage to disappear when you stare at the image for 30 seconds instead of 5 seconds?
Needed for preparing ahead:
- Computer with a color monitor, projector, or a color printer and paper (1)
Needed for each demo or small group at the time of the science activity:
- Computer with a color monitor, projector, or a color printout of Figure 2 below (1)
- Stopwatch or clock that counts seconds (1)
- Colored pencils and paper or a basic computer graphics program
|Figure 1. You need only a few simple household materials to do this fun science activity. You can use a computer with a color printer or a projector in place of a computer with a color monitor. You can use a computer with a basic computer graphics program in place of the colored pencils.|
What to Do
Prepare Ahead (< 10 minutes)
- Students will need to see a color version of Figure 2 below in this activity. You can print the student versions using a color printer (making sure the colors look vivid), or you can have the students look at the student version on a color computer screen. Alternatively, you can do a class demo by displaying Figure 2 with a large color projector.
- If there are any lights that are right next to the computer monitor or the printout of Figure 2, turn them off (but you do not need to make the room dark).
|Figure 2. For this activity students will need to see a color version of this image on a computer monitor, as a printout, or on a large projector.|
Science Activity (10-20 minutes)
- Using a stopwatch or clock, have the students look at the colored image in Figure 2 (focusing on the white spot in the center) for 30 seconds, and then, have them look at the white space to the right of it. Tip: One student can time another student.
- Have students draw the afterimages they saw either on pieces of paper with colored pencils or with a basic computer graphics program. The student version of this activity includes a blank figure where students can draw their afterimages. What are the colors of the three pieces of the circle in the afterimage? The afterimage should look like the right side of Figure 3 below (the left side is the same as Figure 2 and is provided for reference).
|Figure 3. The afterimage should look like the right circle in this figure.|
- Have students look at the colored image in Figure 2 again for 30 seconds and use a stopwatch or clock to see how long it takes for the afterimage to disappear. Then, have students look at the image for only 5 seconds and see how long it takes for that afterimage to disappear. Did it take more or less time to disappear the second time? What was the time difference?
You should see an afterimage that looks like the right image in Figure 3. The color of each piece of the circle in the afterimage is a mixture of two of the three additive primary colors, specifically the two that were not in the corresponding piece of the original image. This is because the cone cells that were used to see the original color became fatigued, and only the two other cone cell types could perceive color when seeing white light. Mixing two of the three primary colors results in the following secondary colors:
- Red and green gives yellow.
- Red and blue gives purple (including magenta).
- Green and blue gives cyan.
The longer you look at a color, the longer the afterimage will persist because it takes longer for the cone cells to recover.
For Further Exploration
This science activity can be expanded or modified in a number of ways. Here are a few options:
- Repeat this activity but have students pay attention to how long it takes for the afterimage of each different color to disappear. Do some colors fade away faster?
- Have all students compare their afterimage drawings. Are they all the same, or are some different?
- Repeat this activity using objects or images that are different colors (colors other than the three primary additive ones). Have students predict what the afterimage will look like, and then, have them check their predictions by doing the activity.