By Kaylin Harder
A positive afterimage occurs when the colours of the original object or photograph are maintained. This happens when you look at a brightly lit area, and then look away but continue to see the image. The original image creates nerve impulses, these nerve impulses will cause the image to continue for a short period of time. After the cells in the retina have been exposed and are energetic and functioning it takes some time for that response to cease. Positive afterimages happen quite often. However, we do not notice them because they can last as little as 500 milliseconds.
A negative afterimage occurs when you see the inverse colour of the original photograph. An example of this is when you stare at a red image for a period of time and when you look away you should see a green afterimage. This is explained in the opponent process theory of colour vision.
The Opponent Process Theory of Colour Vision
The opponent process theory of colour vision is that our colour spectrum is controlled by three receptor cells with opposing actions. The three receptor cells are the red/green, the blue/yellow, and the black/white. According to this theory, these cells can only detect one of the colours at a time. That is why we don’t see red\green or blue\yellow colours.
According to the opponent process theory of colour vision, after staring at a red and green photograph for a long period of time, you exhaust the red and green cone cells. Then, when you look at a blank screen, the red/green cone cells are still exhausted so the only cone cells that can fire are the blue and yellow cells.
Myopia, also known as nearsighted, is when your vision lets you see closer objects more clearly, and further objects become blurry. This occurs when the length of your eye is larger than the optical length. This means that the length of your eye is stretched, it’s longer than normal. This causes the light to connect in front of the retina, not directly in the tissue like it should. Generally, myopia develops at a young age and continues to worsen over time. However, concave lenses in glasses can help to correct myopia because they spread out the light before it reach the natural convex lenses in our eyes, therefore letting the image focus directly on the retina.
Hyperopia also known as farsightedness is the opposite of myopia. It allows you to see objects further away more clearly and the objects that are close appear to be blurry. This occurs because the eyeball could be to short or the eye may have defects such as: a flat cornea or an odd curvature. Hyperopia could be present from birth, however, many people can outgrow the condition. To help correct hyperopia people use convex lenses because they add additional precision so the optical vision of the lenses connects to the retina as well as the natural convex lenses in our eyes.
My first scientific hypothesis was: The effects of afterimages are different for people with and without glasses. After figuring out that my hypothesis was correct and that people with glasses did not see the afterimage as fast as people without glasses, I decided to expand my experiment and procedure. My next scientific question was: what are the variables that affect afterimages? Once I discovered that having or not having glasses varies how people perceive afterimages, I was hoping to determine some of the other possible variables that could affect how we perceive afterimages, and what is the cause.
The variables that will affect afterimages are age, gender, glasses, and brightness.
Younger participants will see the afterimage for a shorter time than older participants because their rods and cones are less developed and get fatigued more easily.
Girls will see the afterimage longer than boys because their physical bodies and mental states mature more quickly.
The afterimage will last longer because it’s easier for their eyes to get fatigued and they won’t be able to focus as well.
The afterimages will last longer when the room is dark because the rods and cones have to adjust to less light.
The independent variables for this experiment were the gender of the participants, their age, whether they had glasses, and whether it was light or dark in the room.
The dependent variables for the experiment were the colour of the afterimage that the participants saw, and how long the participants saw the afterimage.
The control variables for the experiment were the space in which the experiment took place, the time at which the lighting was changed, the equal distance between the participants and the screen, having no other people in the room, and the room being quiet.
For this experiment I used the following materials; I used three different colored images (the birds, the flag, and the watermelon (see Figure 2)). A stopwatch for how long the afterimages lasted, a timer for how long they could look at the image, an iPod for a camera flash for the final test, and a smart board projector to present the pictures.
The procedure for this experiment was as follows:
I first received the consent from my twelve participants.
I explained to them how the experiment would work and asked if they had any questions in regard to the process.I took them into a quiet room with the lights on and showed them the images.
Next, I told them to look at the image for 30 seconds and then look at the blank screen and write down what colours they saw, then wait another 30 seconds to see if the colour had changed.
Finally, they had to write down the time when the afterimage faded.
For the camera flash test, I took a picture with the flash and then told them to close their eyes and write down what colour they saw, then wait another 30 seconds to see if the colour changed.
I measured the time in seconds for how long it took for the colour to fade. I then repeated the experiment without the lights on.
I found that with respect to age, my hypothesis was not correct. Most of the older participants did not see an afterimage and all of the younger participants saw an afterimage for a period of time. For most of the younger participants the afterimage lasted over 10 seconds. 4 out of 4 (100%) of the older participants did not see an afterimage on certain tests, and 6 out of 8 (75%) of the younger participants saw the afterimage for more than 10 seconds.
For gender, my hypothesis was not correct. Most of the time a girl saw an afterimage but it faded almost instantly, and for the male participants the afterimage appeared and stayed in their vision for a long period of time (see Table 2). 6 out of 8 (75%) of the female participants found that the afterimage faded almost instantly (see Table 4).
For people with and without glasses my hypothesis was inconclusive because half of the people with glasses saw an afterimage but it faded almost instantly, as well as the people without glasses half of them saw an afterimage that lasted, however for the other half it faded almost instantly. 3 out of 5 (60%) of the participants with glasses saw an afterimage but found that it faded almost instantly, and 5 out of 7 (71%) of the participants without glasses still saw the afterimage but found that the afterimage faded almost instantly (see Tables 1 and 3).
For the test when the lights were off, my hypothesis was correct. A lot of the participants saw the afterimages stay almost two times as long when the light was off than when the light was turned on. Jane E. Brody said in her article that this is because the rods and cones in our eyes become fatigued easier in the dark, and they use more energy because they have to focus more with less light. 9 out of 12 (75%) of the participants saw that the afterimage lasted two times as long when the lights were off.
I also noticed that for the final test, the camera flash, it lasted almost three times longer than any other test, but only when the lights were turned off.
Also, the colours that people saw varied between genders. Although some of them both saw red, blue, and green, it seemed that one gender saw more of one colour than the other. For example, a lot of the female participants saw the colours blue, green, and red. Likewise, the male participants saw a lot of the colours black, yellow and green.
Based on these results, I found out that there was no significant difference for people with and without glasses. Also, age did not seem to affect how we perceive afterimages. I found out that two of the biggest variables that affected the afterimages was gender and if the room was light or dark.
There are some uncertainties in this study including but not limited to the participants emotional and physical state, distractions, and mood.
This afterimage science experiment could be used to help study the effects and reasons behind colorblindness, and to try and figure out why females were not able to see most afterimages like males could. The study of afterimages could possibly be explored or used for driver safety in the future. For example, rods and cones become fatigued easier at night, and based on this test if you were to retake your driver’s test and they showed you an afterimage and it appeared too quickly they could tell you that it is unsafe for you to drive at night.
In conclusion, I found out that my hypothesis was partially correct based on the four tested categories: age, gender, glasses, and brightness.
My results indicated that younger participants did not see the afterimage for a shorter time than older participants. Girls did not see the afterimage longer than boys as I predicted, in fact boys saw the afterimage longer than girls. There was high variability with glasses so my findings are inconclusive. The afterimages did last longer when the room was dark.
This study suggests that the two main variables that affect afterimages are gender and the brightness of the room.
I would like to thank all twelve of my participants for their help, Kenzie Macdonald, Jennifer Parrott for taking the time to help us with our projects and being our mentor, and the IRC for printing our boards, my sister Kristen and my parents for their support, Steve Dagar for correcting and proof-reading my entire project, the school for providing the opportunity to send us to Ottawa for the Canada Wide Science Fair, and all the teachers and students at the school who encouraged, helped us ,and supported us.
Cherry, K. (2017, December 25). How Afterimages Work. Web. Retrieved from https://www.verywellmind.com/what-is-anafterimage-2795828
Cherry, K. and Gans, S. (2018, April 16). The Opponent Process Theory of Colour Vision. Web. Retrieved from https://www.verywellmind.com/what-is-the-opponent-process-theory-of-color-vision-2795830
Cherry, K. and Gans, S. (2018, July 09). Understanding the Trichromatic Theory of Colour Vision. Web. Retrieved from https://www.verywellmind.com/what-is-the-trichromatic-theory-of-colour-vision-2795831
Cycleback, D. (2003). Eye/Brain Physiology and Human Perception of External Reality. Web. Retrieved from http://www.cycleback.com/eyephysiology.html
Wikipedia. (2018, July 03). Photoreceptor cell. Web. Retrieved from https://en.wikipedia.org/wiki/Photoreceptor_cell.
First Eye Care. (2017, October 18). Nearsighted Vs Farsighted: What’s the difference? Web. Retrieved from https://fecarlington.com/nearsighted-vs-farsighted-whats-difference/
Brody, J. (2007, March 13). Growing Older, and Adjusting to the Dark. Web. Retrieved from https://www.nytimes.com/2007/03/13/health/13brody.html.
About the Author
Kaylin is a grade 9 student who attends the East Three Secondary School in Inuvik, NT. She is optimistic and quiet, and likes to spend time with her friends and family. During her leisure time, she likes to daydream and create stories, and does all sorts of watercolour painting and modern sewing. She creates many things, from quilts and pillows to dresses and trench coats. Both she and her older sister play the piano and this summer she is beginning to learn and practice the violin.