Science project

What Affects the Appearance of Afterimages?

I thought age would affect the length of time it took for an after-image to appear. Age didn't appear to a factor but subjects who wore glasses took almost twice as long to see and after-image. It was very interesting that twenty percent of all subjects did not see any sort of an after-image.


What variables affect the appearance of an after-image in humans?

Background Research

Cones are cells in the eye that respond to color. Rods are cells in the eye that respond to light. S-cones respond to the color blue, L-cones respond to the color red, and M-cones respond to the color green. An after-image is an image that is generated in the eye and stays with you even after you have stopped looking at the object.

An after-image is the negative of the image you were looking at. This happens after staring at an image for thirty seconds or more. When you turn away the cones in your eyes become over tired from looking at the same image. Then these cones start to relax and the cones that produce the opposite color take over and the opposite color appears on your retina, this is the after-image.

After-images can be seen anywhere. If a flash from a camera goes off, then a blue-yellow shape of the flash appears. When a person looks at a green object for a long period of time then turns away they will see red. Bright light might stay for up to ten seconds but color will only stay for a couple seconds, although the bright light hurts the eyes for a long time.


I predict that age will affect the length of time it takes for a subject to see an after-image. I think that the older the subject is the longer it would take them to see an after-image.


  • Two sheets of white paper
  • One sheet of green construction paper
  • One sheet of red construction paper
  • Glue
  • Scissors
  • Stopwatch
  • CBL-Unit (part of the light probe)
  • Texas Instruments 83 calculator (part of the light probe)
  • Data sheets
  • Subjects (human)


  • Using construction paper cut out one green apple shape and a red leaf shape. Glue to a piece of white paper with the apple in the center and the leaf to the upper left side.
  • Find a person who agrees to being tested.
  • Set up the CBL unit and calculator to read the amount of light.
  • Adjust the amount of light in the room so it reads between .087 and .097 mW.
  • Explain to the subject the procedure and what they will see.
  • Have the subject stare at the apple image for one minute. To time the subject use a stop watch.
  • After one minute have the subject look at the white sheet of paper and indicate as soon as he or she sees an after-image, this is timed using a stopwatch.
  • Record the subject's age, gender, whether they wear glasses, time to see after-image, and the amount of light.




I observed that subjects who didn't wear glasses saw the after-image in half the time. Subjects who were older took longer most times, whereas young subjects typically saw an after-image quicker. Females saw an image a second sooner than most males. I noticed that people in the same light typically saw an image at the same time.

Often subjects would say "neat" or "cool" when seeing the after-image. Many subjects were doubtful of seeing the image and were extremely amazed when they did.


My experiment indicates that there is a relationship between the age of a subject and the time it took to see an after-image. The older subjects generally took longer to see an after-image, whereas younger subjects saw the after-image sooner. Although there were some exceptions and people who did not see an after-image.

I think it took older subjects longer to see an after-image because their rods and cones are worn and don't react as fast as a younger person's does. I accept the hypothesis. I feel that a larger sampling of people would prove my hypothesis even more.

I found it very interesting that twenty percent of subjects saw no after-image. This could be explained if subjects didn't understand my instructions. Also I believe that subjects who mentally decided that this was impossible did not see an after-image because they didn't want to.

A major factor (according to the graphs) was vision. Subjects who didn't wear glasses saw the after-image in half the time it took subjects with glasses. I think that this because most of the subjects tested wore glasses. Most people who wear glasses are old, and as I stated above older subjects took longer to see and after-image.

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