Athabasca Falls are a small set of waterfalls situated in Jasper National Park. Before we get to the actual waterfall, let us venture a little further up the river, where the water is shallow, smooth and calm. The riverbank is lined with coniferous trees, suggesting a mountainous/sub-alpine Eco-district, and adding to the effect of calmness and peace we can sense when in that area. If we are really quiet, faint bird calls can be heard, and sometimes, the occasional chipmunk will venture too close and then scurry away, too frightened to come any closer. Moving along the river approaching the falls, we observe more and more rocks and rugged terrain, as well as fewer trees along the riverbank. The closer we get to the actual waterfall, faint sound that used to sound peaceful becomes a sound that announces something dangerous, like suspenseful music in a mystery movie. The falls themselves are rough and uncontrolled. The Athabasca River thunders through a narrow gorge where the walls have been smoothed and potholes are created by the sheer force of the rushing water carrying sand and rock, eroding everything below that happens to be in its path. Due to the fact that the water falls vertically down, and the rocks adjacent to the falls are composed of horizontal strata, if we stare at the water for a certain amount of time and then transfer our attention to the rocks, we get the impression that the rocks are moving in a wavelike pattern. Looking below the waterfall, the spray of the waterfall creates a sort of mist in the air, which then splits the light into a spectrum, creating rainbows in various places at the foot of the waterfall. This seemingly simple, majestic and powerful display of water is in fact a series of molecular bonds, laws of physics and optical illusions which interact in complex manners, creating the fascinating effect we can observe.
Memo Report
Optical illusions are caused by many factors, and each one can cause a different effect. In this paper, we will see the causes and effects of the Afterimage effect, and how it can be explained.
What is an illusion?
An illusion is an impression of a real stimulus which does not agree with any other of the senses, whether physical or psychological. for example, the Athabasca falls illusion causes us to see a rock wall, about 15 metres high, seemingly waving like water. Visually, this is a fact, but if we were to touch the wall during the effect of the afterimage, we would notice that is not moving at all. We have to realise that illusions are not real, and can be caused by many things, overexposure is merely one of these things. By playing with the senses, one can achieve almost any type of illusion, what that is depends on what you do1.

The illusion in the eye:
A big part of optical illusions are caused by the eyes alone. What we call sight is simply a set of signals interpreted by the eye and transferred via the optic nerve to the brain. This occurs in 7 steps, according to Matthew Luckiesh who wrote a book on optical illusions in 1922. The structure of the eye can be simplified for our purposes by saying it has a roughly spherical shape, with a hole in its front, called PUPIL, surrounded by the iris, and a specialized nerve located at the back of the eye called RETINA. Thanks to these two structures (along with the rest of the eye’s components), humans are able to see. However, certain defects in the eye (which all eyes have) cause us to experience optical illusions. These defects are known as the spherical and chromatic aberrations3. The eye sees thanks to the conversion of light to a nervous impulse that is transmitted from the photoreceptors in the retina of the eye to the optic nerve and then to the occipital lobe of the brain. However, if these receptors are overstimulated they tire, and cause an “afterimage”, which is a temporary impression of the image in the visual field even after we are no longer looking at it. These defects, along with a person’s ability to use binocular vision enable us to experience many psychological as well as optical phenomena3.
Afterimage effect:
One of the most common afterimages is the bright glow that seems to float before someone’s eyes after looking directly into a light source for a certain amount of time. There are two types of afterimage, positive and negative, but while negative afterimage can be explained scientifically, the positive counterpart cannot. The illusion caused by the Athabasca falls is a perfect example of Negative Afterimage. A negative afterimage is a phenomenon caused by overstimulation of the photoreceptors in the eyes. This causes the receptors to tire, and therefore, when the object we are looking at changes, they do not respond as quickly, giving us a double perception, that of the previous object and that of the object we are currently looking at. In the case of the Athabasca falls, what happens is we overstimulate our photoreceptors in our retina to a bright light, with a constant downward movement of the water during a certain amount of time. When we then shift our gaze from the vertically moving, light reflecting surface to a darker, horizontally striped surface (the rocks) we perceive the rocks as moving in wavelike patterns for a short period of time, This is caused by the photoreceptors in our eyes not having enough time to recover from the overstimulation of the bright light reflected off the waterfall, and are still slightly stimulated as what we are looking at changes4.
Conclusion
We have observed the causes and effects of the afterimage effect, and can state that it is a naturally occurring phenomenon due to the overstimulation of our photoreceptors in the eyes, as well as bright light and fatigue of the cones and rods in the retina.
Here is an example of negative afterimage: http://en.wikipedia.org/wiki/File:Illusion_movie.ogg
(Stare at the moving picture for 30 seconds(try not to blink, then look at a wall.(Athabasca falls effect))

References:
Anonymous source “Illusion” [internet] http://www.howstuffworks.com/illusion-info.htm
Glasser DM, Tsui, JMG, Pack CC, Tadin D. Perceptual and neural consequences of rapid motion adaptation. (November 8 2011) Proceedings of the National Academy of Sciences of the United States of America Volume 108 Issue 45: E1080-E1088
Matthew Luckiesh “Visual Illusions: Their Causes, Characteristics and Applications” “chapter 2, The Eye” 1922.[internet] http://www.visualillusion.net/
Wikipedia Staff “Afterimage” [internet] http://en.wikipedia.org/wiki/Afterimage