Student Guide: Discovering the Colors Behind Afterimages

What happens when you look at one color for a long time? Why do you see the afterimage that you do? Find out how the colors of afterimages are made using a computer, a stopwatch or clock, and colored pencils.

Useful Vocabulary

• Cone cells: Cells at the back of the eyes that let us see colors. There are three different types of cone cells, and each responds to red, green, or blue light. If you look at one color for very long and then look away, the cone cells temporarily do not see that color because they are worn out.
• Primary colors: Primary colors are usually a set of three colors that can be mixed in different amounts to make any color. The primary additive colors, which are used for computer screens, are red, green, and blue. (The subtractive primary colors red, yellow, and blue are often used in art, such as painting and printing.)
• White light: White light is made when the light of all three primary additive colors is mixed together in equal amounts.
• Afterimage: An optical illusion, or "ghost image," that you see after you are no longer looking at the original image. It commonly happens after you look at a bright light.

Materials

To do this activity you will need:

• Computer with a color monitor, projector, or a color printout of Figure 1 below (1)
• Stopwatch or clock that counts seconds (1)
• Colored pencils and paper or a basic computer graphics program

Directions

1. For 30 seconds, look at the colored image in Figure 1 below, focusing on the white spot in the center. Another student may help you keep track of the time.

Figure 1. In this activity, you will stare at the colored image on the left and then quickly look at the blank space on the right to see the afterimage.

2. Immediately after looking at the image, look at the white space to the right of the image. What do you see in the white space?
3. Use colored pencils or a basic computer graphics program to draw the afterimage you saw. You can use the blank space in Figure 2 below to draw the afterimage.
4. What are the colors of the three pieces of the circle in the afterimage? Why do you think each piece of the afterimage has the color it does? Hint: Think about which cone cells you use to see each color and how the primary additive colors mix together to make other colors.
5. Why do you think the afterimage disappears?

Figure 2. You can draw the afterimage you see in the blank space in this figure.

6. After you draw the afterimage, look at the colored image in Figure 1 for 30 seconds again. Then, immediately look at the white space on the right of the image. See how long it takes for the afterimage to disappear after you are done looking at the original image. Another student may help you keep track of the time. How long did it take to disappear?
7. Look at the colored image in Figure 1 again, but this time look at it for only 5 seconds before looking at the white space on the right. Again see how long it takes for the afterimage to disappear. Did it take more or less time for the afterimage to disappear this time? What was the time difference? Why do you think it took more or less time?

Downloads and Links

• Discovering the Colors Behind Afterimages Student Guide PDF