Seeing is believing
If we wish to understand how vision works, we have to look at not only the explanations provided by physics, but also at cognitive science. On a totally scientific basis this science now substantiates the millennium-old scepticism of philosophers as to whether an objective reality is at all accessible to human beings. Is what we see reality, or is it "only" a fabrication of our brains? Today, it has been proven that the latter is the case. For our vision, this means: we cannot see a real world objectively.
Some key discoveries made by cognitive science make this clearer: the information transmitted by the sense receptors of the eyes to the brain is devoid of meaning. Only in the brain is any meaning or significance assigned to these signals on the basis of experience. What human beings see is therefore their own construct of reality. In the words of cyberneticist Heinz von Foerster: "The environment we perceive is our invention."
Do you really see it?
Here we see a few examples showing how our brain constructs its own reality.
The small square has the same colour in all of the larger squares. Nevertheless, the small square seems to get darker and darker from left to right due to its contrast with the large squares.
The optical illusion with the Hermann’s grid.
Our brain makes us see grey dots in the corners of the white lines, but they do not really exist. This also happens when you see the grid in contrasting colours.
The pink circle surrounded by large, black dots seems to be smaller than the pink circle between the small black dots. In reality, however, the two pink circles are exactly the same size.
The information provided by the two eyes is first processed in the brain into one three-dimensional image. Only the perfect interaction of the eyes allows a perfect 3D image and natural spatial perception, permitting the correct assessment of the size and distance of objects.
You can find an amazing collection of optical illusions on the website of Prof. Dr. rer. nat. Michael Bach from the University Eye Clinic in Freiburg, Germany:
Optical Illusions & Visual Phenomena