The eye and brain are both easily fooled. There are many kinds of optical illusions. Some involve colour and/or contrast. Others use perceived shapes. Still more require the image to move to create the illusion.
A simple example is to hold your finger in front of your face. You will seem to be able to see through it. But if you close one eye, your finger will become solid. For other illusions, the brain is used to perceiving things a certain way. A railroad track seems to be coming together in the distance. Knowing that the tracks are straight and parallel doesn’t prevent this automatic depth perception in real life, while drawing it on paper gives that illusion of distance and size because we are used to seeing things that way. Also, because of how the retina works, objects can seem to change in colour, disappear or even move, or can be seen clearly even after the eyes are closed.
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