SENSE OF SIGHT: THE PICTURES IN YOUR HEAD
by Chris Gummert
The first camera ever invented, dating back to roughly 400 B.C.E., is called the camera obscura. It's simply a wooden box with a hole in it. But it momentarily captures images. Light coming into the darkened box projects an upside down picture on the rear wall of the camera, a picture of whatever the hole is pointing toward. Point it at a flower, you get an upside down flower picture. Tree; upside down tree. Pineapple cake...you get the picture. Not too bad for a box with a hole and exactly zero moving parts.
Our sense of sight works in pretty much the same way, but with more moving parts and a lot less wood. Simply put, our eyes use reflected light to create an image. Light enters the eye through a small hole called the pupil. It's located in the center of the iris, the colored part of the eye. Unlike the camera obscura's single hole though, the pupil will change in size depending on the available light. In a dark room the pupil opens wide to take in as much light as possible. In bright light it becomes smaller. Next the light passes through the lens of the eye. The lens serves to focus the light onto a spot on the back of the eye called the retina.
Think of the inside of the eye like a tiny movie theater, but much cleaner. The pupil and lens are the projection booth, the light is the movie and the retina is the screen. The retina is coated with specialized cells called rods and cones. Rods judge how intense the light is, while cones figure out what colors are present in the picture and send that information to the brain. The firing of rods and cones is extremely fast, but not quite fast enough to be constant so our brain is supplied with a series of rapidly-updated still pictures. In between pictures your mind retains a brief after-image that overlaps with the next. That overlap is enough to trick your mind into believing it is seeing constant, fluid motion. This is also how animation works.
So from the moment you open your eyes in the morning until you go to bed at night your brain is watching the world's longest running cartoon.