The Trick Behind Reality's Disappear/Reappear Act

Have you ever been blinded by a bright light after stepping out of the dark? Have you ever noticed the details of your dark bedroom at night slowly start to creep into focus after initially not being able to see anything? Of course, it is not reality itself that is disappearing and reappearing, but your perception of it. Why does this happen?

These phenomena result from the biological mechanisms that govern the absorption of light by the eye. In the back of the eye, on the retina, is an array of photoreceptors with pigments that do the job of absorbing quanta of light. In humans, there are two types of photoreceptors - rods and cones. Rods and cones differ in several functional and physiological aspects, but of interest here is that they each process different levels of light intensity. The sensitive rods permit us to see dim light ranging in intensity from 10^1 - 10^5 under dark conditions (referred to as "scotopic" vision) while cones process bright light ranging in intensity from 10^6 - 10^11  ("photopic" vision). It is a common misconception that the change in pupil size accompanied by a change in light intensity is the major contributor to the eye's ability to process light of different intensities. However, the human pupil undergoes a maximal 16x increase in area as it changes from 2mm to 8mm in diameter, which is of the magnitude 10^1, whereas our actual range of perceptible light intensities is of magnitude 10^11. Thus it is our rod/cone photoreception that allows for our so called "duplex" retina capable of processing both dim and bright light of a range of intensities.

When a quantum of light strikes either a rod or cone photoreceptor, it initiates a complex enzymatic cascade that begins the processing of light by the brain. To get the process started, part of the pigment in the photoreceptor actually changes shape as it is "isomerized" into a different chemical form. Once the pigment has been isomerized, it can no longer absorb another quantum of light. However, in order that we are able to view different visual events over a short period of time, a reverse process of regeneration is soon after initiated to reconvert the pigment back into its original form. At that point, the pigment can absorb a new quantum of light and begin a new enzymatic cascade. The process of isomerization and regeneration occurs on a timescale of seconds and minutes. With this knowledge, we can understand why the phenomena in question occur.

If you have been sitting in a well-lit room for a while, and suddenly enter a dark room, initially you see absolutely nothing. This is because your sensitive rods have not had enough time to regenerate yet. In about 35 minutes, all of your rods will have had time to regenerate, and you will have become completely "dark adapted" and capable of seeing fairly well. The same happens with your cones when you are faced with a very bright light, as they become isomerized and are rendered incapable of absorbing any further light for a short time.

This is all yet another example of how the limitations of our biology create illusions in our perception that misrepresent reality.