Imagine that you’re sitting outside on a sunny, cloudless day. You look up at a flock of sparrows that’s passing through your backyard, and as you follow them you accidentally look at the sun. The intense light blinds you for just a second, and as you cast your gaze back at the ground you notice that there’s a strange, glowing blot on your vision. It passes quickly, and you know it will, because you’ve seen it many times before. This is an afterimage.
An afterimage is exactly what the name implies. It is the lingering image of an object, recently viewed, that has temporarily imprinted itself on the eye. Consequently, wherever the viewer looks they will see that image until it wears off. Afterimages are the result of looking at a source of light.
What Causes Afterimages?
Saying that light causes an afterimage isn’t quite accurate. Light bounces off objects all the time, seldom causing the afterimage effect. More important to creating an afterimage is an overexposure to one source of light.
Looking at the sun or directly into a light bulb will create an afterimage, but less damaging experiments are probably wiser. Open a graphics program, such as MS Paint or Adobe Photoshop, and create a black circle on a white background. Then stare at the centre of the circle. After about ten seconds a smaller, white circle will begin to appear inside the black circle. Why does this happen?
The key lies in photoreceptive cones in the eye. Cones take in light and convert it to electrical activity that’s transmitted to the brain for processing. There are three different kinds of cones – blue, green and red – the combination of which creates the many colours. These cones are also responsible for afterimages.
Why? Because cones can become fatigued by continuously taking in light from the same source. If a set of cones is active for too long it will fail to respond for a short period of time. Looking away from the object will result in an afterimage as the remaining two sets of cones continue to process the original image. Once the fatigued set of cones comes back on line vision will return to normal, and the afterimage will vanish.
How Long Does an Afterimage Last?
Afterimages are short-term phenomena. Typically the cones in the eye will return to normal after several seconds of fatigue. Constant exposure to strong sources of light, notably the sun, can cause afterimages to last much longer. This can be indicative of damage to the cones, however, and it’s highly recommended not to concentrate on bright lights.
How Are Afterimages Used?
Biologically, afterimages are just a by-product of desensitization. The fact that cones become desensitized by concentrating on an unchanging stimulus is an important survival mechanism, because things that are changing are usually more important than things that are not.
The predictability of afterimages has also resulted in their use in optical illusions and art pieces. Placing black patterns on white backgrounds, or vice versa, can trick the eye into seeing things that either don’t exist, or which are smaller or larger than they appear. These optical illusions can also be used to create moving afterimages.
Afterimages can be fun to play with, but it’s important not to strain the eyes too much. Light is damaging in sufficient amounts, and can impair eyesight significantly. Afterimages can even become permanent, prominent fixtures in palinopsia sufferers. Always be careful to avoid looking directly at extreme sources of light.