Afterimages: The Colorful Tricks Eyes Play
IntroductionHave you ever wondered how visual illusions are made? When we see special effects in movies, or a magic show, we often witness illusions that challenge our ability to correctly perceive things. One way in which our eyes play tricks on us is through afterimages. Afterimages are the images you see after staring at an object for several seconds and then looking away. In this science activity, you will look at afterimages to reveal the secrets of how your eyes see color.
- Computer with a color monitor or a color printer and paper
- Stopwatch or clock that shows seconds
- Optional: Colored pencils and paper or a basic computer graphics program
- To do this activity, you will need to see the image below of the colored circle. Because of this you will need to have access to a computer with a color monitor to show the image, or you can print it out on a color printer.
- If there are any lights right next to the computer monitor or the printout of this activity, turn the lights off.
- Look at the image of the colored circles (focusing on the small white spot in the center) for 30 seconds.
- After staring at the circle for 30 seconds, look at the white space to the right of it.What do you see? How are the colors in each part of the afterimage different from the parts of the original colored circle?
- Optional: You can use colored pencils and paper or a basic computer graphics program to draw your results.
- Thinking about the primary colors (red, blue, and green) and their secondary colors (yellow, purple/magenta and cyan), and how afterimages are caused, see if you can explain your results.Why do you think you see the afterimage colors that you do?
You should have seen an image that is similar in shape and size as the original image but different in color. The image you saw on the white surface is called an afterimage.
Afterimages occur because of the way we see color. Your eyes use three different groups of cells (specifically called cone cells) to see color and each group only sees red, green, or blue. To perceive white, all three groups of these cells—the red, the green, and the blue cells—need to respond. If you look at a red object, the red cells respond. If you stare at the object for some time and then quickly look at a white area, you will see an afterimage that is the same size and shape, but it has a cyan (blue-green) color. This happens because the red cells are worn out from being used, so they do not respond. You are temporarily left seeing with only your green and blue cells, and you see a blue-green color instead of white. The same process happened to your eyes in this activity, but it happened not only with the red cells but also with the blue and the green cells. The area where blue cells got worn out, you were temporarily left seeing only with the green and red cells. Because you perceive the mixture of red and green as yellow, the area that was blue in the original circle looked yellow in the afterimage. The green part of the original circle looked purple, or magenta in the afterimage because the green cells got worn out in this area and temporarily, only the red and blue cells responded. The mixing of red and blue light is perceived as purple, or magenta, so that part of the circle looked magenta in the afterimage.
The afterimage disappears after several seconds because the fatigued cone cells recover and become active again. With all the cone cells active, all three types of cells respond to the white surface, and you see the white region normally.
If the red cone cells in our eyes see red, the blue cone cells see blue, and the green cone cells see green, how is it that our eyes detect other colors like yellow or white? It turns out that all the other colors we see are a result of the different cone cell types responding together. For example, if you look at a yellow image (which is a mix of red and green) both the red and green cone cells respond. Your brain interprets this as yellow. The exact shade of yellow depends on the balance of red versus blue cone cells. To see white, all three types of cone cells must respond.
You may have frowned when reading that the primary colors are red, green and blue, and thought mixing red and green produced a dark brownish color, definitively not yellow. If so, you were probably remembering how you mix different colored pigments in paints, crayons or other art supplies. Light follows an "additive" color model, while pigments follow a "subtractive" color model. As shown in the figure, red, green, and blue light combines to make white light. Mixing two colors of light at a time produces yellow, magenta, or cyan colored light. That is very different from mixing colors of paint or ink.
You can predict the color of the afterimage for most colors—it is the complementary color to the color of the object (using the additive color model). If the original image has the additive primary colors like the one in this activity, the mixture you see in each part of the afterimage is made up of the two primary colors that were not in that part of the original image. Mixing two of the three primary colors results in the following secondary colors: red and green gives yellow (which you should have seen in the top left part of the afterimage circle), red and blue gives purple, or magenta (which you should have seen in the bottom part of the afterimage circle), and green and blue gives cyan (which you should have seen in the top right part of the afterimage circle). Try to stare at a circle with yellow, cyan, and magenta and find that a circle with the primary colors appears as afterimage. Staring at yellow wears the red and green cone cells out, so when you switch to looking at white, only the blue cone cells respond. In a similar way, staring at cyan creates an afterimage that looks red, and staring at magenta creates an afterimage that looks green. Blue and yellow are complementary colors, so are cyan and red, and magenta and green. As the complimentary color is the color that when combined with the given color makes white, the afterimage when switching to looking at white will always show the complimentary color.
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For Further Exploration
- Time how long it takes the afterimage to disappear. Then look at the colored circle for only five seconds and again time how long it takes that afterimage to disappear. Did it take more or less time the second time?
- You could try repeating this activity but this time pay attention to how long it takes for the afterimage of each different color to disappear. Do some colors fade away faster?
- Try doing this activity with several different people and have each person draw their results. Are they all the same, or are some different?
- You could try this activity again but this time use objects or images that are different colors (colors other than the three primary additive ones, which were used in this activity). Can you accurately predict what the afterimages look like?
- Can you predict what color the after image will be when looking at a cyan page after adapting to blue? What about adapting to green or red first? Never forget to test your hypotheses.
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