Your Eyes Create Borders That Do Not Exist

Your brain constantly tricks you by adding fake shadows to images. When two different shades of gray touch, your eyes manufacture an optical illusion called Mach bands to sharpen the boundary.

This phenomenon happens because your visual system is wired to exaggerate contrast at edges to help you spot objects more easily. It creates a dark stripe and a light stripe where neither actually exists on the page. Even if you measure the gray levels with a sensor, your brain will stubbornly insist the edges look brighter or darker than the rest. It is a perfect example of how your eyes prioritize visual clarity over total physical accuracy.