How the Blind Spot works

You just found your blind spot. Everyone has one in each eye, and almost nobody ever notices it.

A spot that cannot see

The back of your eye is lined with more than 100 million tiny light sensors, called rods and cones. Their signals travel to your brain through a thick cable of nerve fibers, the optic nerve. Where that cable leaves the eye, there is no room for rods or cones. That small patch, the optic disc, cannot see anything at all.

Your right eye, seen from above. Light from the dot crosses inside the eye and lands on the blind spot.

Why the dot vanished

The blind spot sits about 15 degrees toward your ear from wherever you look. With your left eye closed and your right eye on the +, the dot was off to that side. At just the right distance, its light fell exactly on the optic disc. Nothing there could feel it, so your brain never heard about the dot.

Why you never notice it

With both eyes open, each eye sees what the other one misses. Even with one eye shut, your brain does not show you a dark hole. It fills the gap with whatever is around it: white screen, a wall, the room behind your friend. That is why the dot melted into the screen, and why a whole head can vanish from three steps away.