Some descriptions of this phenomenon use the term “visual” instead of optical. It may be best to determine the difference between these two terms, if there is one.
First, the word “optical” actually refers to the field of optics or the science of optics. This is an area of science that is meant to assist sight or correct defects in vision. Vision or sight is one of the senses that reveals objects in the world around us, with all of the associated light levels, colors and motion.
So an optical illusion as most people know it would better be described as a visual illusion. We see something, or think we see something. That’s visual information. With the correct combination of light, color and image we might “see” a visual effect that isn’t a real object. This is one form of optical illusion or visual illusion.
In almost every case, when several people are standing next to a large tree, in good light, everyone is able to agree that they see a tree. With an optical illusion, however, some may not see what was intended at all, or they may interpret it differently than the person next to them does.
It all has to do with how the information gets to the brain and how it is interprested by the brain. One simple example, that is not a magic trick or intended to confuse an audience, is the image of bright light that remains after someone has looked at a very bright bulb or fire. This could be described as an optical illusion, in that the image or memory of the image remains after we have actually seen this light.
Another example would involve a person looking for a long time at something that is blue. When that person turns to look at a blank wall, the image of something yellow generally appears. This is an illusion involving blue’s complimentary color.
Basically, the eye looks at an image and the brain does not interpret it correctly. One argument in science and philosophy focuses on how people see things on a daily basis. A building might look rectangular or at least that’s the way the brain interprets it. But if we could see the building in great detail, or from very close range, the brain might interpret the slight curves and imperfections of the building’s surface and edges.
Some optical illusions or visual illusions are intended. A magic show might be said to include a lot of these illusions, involving quick motion, for the most part. Some optical illusions involve patterns on paper that the brain cannot resolve satisfactorily. In one simple example, a large square contains a number of smaller squares that are separated by white lines. But when staring at this, a human being cannot help but see gray in the white lines. That’s just the way the brain works when confronted by such contrasts. This goes back to the old show business saying of “Now you see it, now you don’t.” Or, do you?