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Are Your Eyes Playing Tricks on You? Discover the Science Behind Afterimages!



Are Your Eyes Playing Tricks on You? Discover the Science Behind Afterimages!

www.sciencebuddies.org/science-fair-projects/project-ideas/HumBio_p021/human-biology-health/eyes-playing-tricks-afterimages

Experimental Procedure

There are three parts. Each part should take less than 1 hour.

In Part 1, you will stare at a colored object for various amounts of time, then measure how long it takes for the afterimage to disappear each time. You will then graph the data as Cone stimulation (how long you stared at the colored object) vs. Afterimage persistence (how long before the afterimage disappears). Things to think about: Is there a minimum cone-stimulation time required to produce an afterimage? Do longer cone-stimulation times always cause longer afterimage times, or does the effect level out past a certain time?

In Part 2, you will test whether or not afterimages made by three different colors disappear at the same rate (this measures cone cell recovery).

In Part 3, you will test which color produces an afterimage in the least amount of time (this measures the rate of cone cell fatigue).

Part 1: Duration, Shape, and Color of an Afterimage

  1. Look at Figure 1, below. Have your helper get ready to time you for exactly 30 seconds. You should say "start" when you begin looking at the object, and the helper can say "stop" when 30 seconds have elapsed.
  2. Stare at the red circle for 30 seconds. Try not to blink. When 30 seconds are up, look at the white space in the square to the right. Describe the size, shape, and color of the afterimage in your lab notebook. Note: It is important to view this on a bright monitor with clear colors, or if you printed it out, that the color is vivid.
  3. Turn off any lights near your computer or the printed-out image. Stare at the red circle for 30 seconds again, but this time, record how long the afterimage is visible in your lab notebook. Have your helper use the stopwatch to keep track of how long you are able to see the afterimage. You might want to say "start" as soon as you begin looking at the white space and keep saying "I still see it" repeatedly until it disappears, then say "it's gone" so your helper can stop the stopwatch and you can note the time. Why does the afterimage disappear?
  4. Repeat step 3 two more times, for a total of three trials, and record all data in a data table in your lab notebook, like Table 1, below. Note: Give your eyes a minute or so to rest between every trial.
    Human Biology  Science Project red circle figure

    Figure 1. This is the first afterimage test for this science fair project. View the red circle on the left for 30 seconds, and then note the afterimage when you look in the white space to the right.

    Now let's see how changing the amount of time you look at the red circle ("cone stimulation" time) changes the time that the afterimage is visible ("afterimage persistence"). How do you think the length of time the afterimage persists will vary?
  5. Repeat steps 3-4 four more times, changing the amount of time you look at the red circle each time. Look at the red circle for 5, 10, 20, and then 60 seconds before you look at the white area (you already completed the 30-second trial in steps 3-4).
  6. Record how long the afterimage persists after each time in your data table.

      Afterimage Persistence (seconds)
    Cone Stimulation
    (seconds)
    Red Circle
    Trial 1
    Red Circle
    Trial 2
    Red Circle
    Trial 3
    Red Circle
    Average
    5    
    10    
    20    
    30    
    60    
    Table 1. In your lab notebook, record your data in a data table like this one.

  7. Calculate the averages and insert that data in the last column.
  8. Graph the data. Put the number of seconds you looked at the object on the x-axis and the average time the afterimage persisted on the y-axis. Call the x-axis Cone Stimulation and the y-axis Afterimage Persistence, or choose your own axis names. Include the units (seconds) on the graph.
  9. What was the maximum amount of time that the afterimage persisted?
  10. If you stare at the object for longer periods of time, beyond 30 seconds (say 60, 90, and 120 seconds) does the persistence time of the afterimage also keep increasing? Explain your results in terms of the level of fatigue of the cone cells.

Part 2: Determining Cone Cell Recovery Rates

Human Biology  Science Project three-color circle figure

Figure 2. Use this three-colored circle to compare cone cell fatigue and recovery times.

  1. Turn off any lights near your computer or printed-out image. Stare at the three-colored circle in Figure 2 for 30 seconds and then look at the white space to the right. Using the same method as in Part 1, have your helper time how long you see the afterimage and record the data in your lab notebook.
  2. Also describe the colors of the afterimage in your lab notebook. In your descriptions, call the three sectors Left, Right, and Bottom. This avoids confusion about the colors.
  3. Use colored pencils or a computer graphics program to draw the afterimage.
  4. Repeat Part 2 steps 1-3 two more times, for a total of three trials, recording your data each time. And don't forget to give your eyes a break between each trial.
  5. How do afterimage persistence times compare for the blue, red, and green regions? Do the three sectors disappear at the same rate, or at different rates?

Part 3: Determining Cone Cell Fatigue Rate

  1. Look at the three-colored circle in Figure 2 very briefly, for 2 or 3 seconds, then look at the white space. Record what you see or don't see in your lab notebook.
  2. Repeat Part 3 step 1 with different times (1, 2, 3, 4 seconds) to determine the minimum time you need to form an afterimage.
  3. Did the afterimage include all three segments? Or did only one of the colors make an afterimage? If so, how would you interpret this in terms of which cone cell fatigued the fastest?
  4. Record your observations in your lab notebook.

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