Many of us were taught in school that the eyes don’t technically see images, but rather collect light for the brain to process as an image. Remember the lesson of your eyes detecting things upside down? The visual cortex at the back of your brain is where it’s turned right side up so you can perceive it clearly. For that to happen, your eye transmits the light to the cortex through its optic nerve. Glaucoma is a chronic disease that involves damage to this nerve, and it can gradually grow worse until it causes complete loss of sight.