What is an Optical Illusion?

The simplest way to describe an optical illusion is that it is a “trick” that our eyes play on us. We seem to see something that isn’t really so. Or we may be able to see the same object in two completely different ways.

If our eyes are functioning properly, and they are instrument for seeing exactly what is before functioning properly, and they are instrument for seeing exactly what is before us, how can the play  “tricks” that our eyes on us? Here is what makes it possible.

Vision is not a physical process. It is not like photography, for instance, which works mechanically. Vision is really a psychological experience, because it is not the eyes that see, but the brain! The eyes are mechanical instrument for receiving impressions.


But when those images reach the brain, a judgement takes place. The cells of the brain have to decide what they decision? One of the important things is the work that the eye muscles have to do in order to see a thing.

In judging distances, angles, and the relationship of things in space, our eyes have to move back and forth. Our brain says that our eyes have traveled a certain distances because the brain has an idea of the amount of energy and the time it took for our eyes to move back and forth.

So now we have the possibility of one kind of optical illusion. Suppose there are two lines the of equal length, but one is vertical and one is horizontal. The horizontal line must be shorter!

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