Eyes are our most important sensory organs because we usually only believe something when we actually see it with our own eyes. The Greek philosopher Plato explained why we should not always trust our eyes. He was right, of course, because our eyes cheat us more often than we think.
Contrast is essential for our eyesight. The text in the left banner is not immediately visible. This is because the contrast between the color of the text and the color of the banner is inadequate. The text in the right banner is easy to read because there is ample contrast. Without contrast, we would simply not see as much.
The small square is the same color in all the big squares. However, the small square seems to get darker from left to right because of its contrast with the big squares.
In this paved wall the horizontal lines seem to be moving towards each other. But they are totally parallel. The squares are perfectly square.
The Hermann grid illusion. Our brain makes us see gray spots on the corners of the white lines, but they do not actually exist. This also happens when you see the grid in contrasting colors.
The pink circle which is surrounded by the big black spots seems smaller than the pink circle in between the small black spots. However, in reality they are exactly the same size.
The horizontal lines are the same size. However, the upper one seems to be longer. We are used to seeing depth. And we think the slanting lines going into the horizon are parallel (like a rail track). Something that is the same size in the distance seems bigger for the brain than the same object in the foreground.
This is a well-known geometric illusion which was designed by F.C. Müller-Lyer. The horizontal lines are the same size in both images. However, the horizontal line in the upper image seems to be bigger/longer.
At first sight, the slanting lines do not appear to be each other’s extension, but when you put a ruler next to them, you soon see that it is in fact the same line. In the second image the dotted line shows where most people think the line should end up.