Optical illusions occur when our eyes trick us into perceiving something that is not physically present or some movement that does not exist. It’s only that your brain is getting mixed signals from your eyes, which is causing confusion.
The human brain is analogous to a supercomputer that is unable to see, despite its high level of intelligence. The eyes are the sole means through which the brain can perceive the world. The issue is that our eyes are only capable of using a limited vocabulary to describe the world around us. And at other times, our brain is unable to properly interpret what the eyes are attempting to convey to it.
The language that our eyes and brain use to communicate with one another is quite straightforward. In order for our brain to make sense of this language, it has to make some reasonable guesses based on the obvious cues provided by our eyes. Those predictions are, for the most part, accurate (For instance, I can observe that the door appears to be about this size and that the sunlight falls on it in this direction; hence, my brain is interpreting these simple indications and estimating that the door is around one meter away.). Our brain is able to comprehend what the eyes are trying to convey to it the vast majority of the time, therefore generally it is not a concern.
However, there are situations when the brain makes an incorrect assumption. And at this point, we are presented with an optical illusion. Optical illusions occur when our minds and eyes attempt to communicate with each other in straightforward language, but the meaning of the message becomes muddled as a result.