Your retina is part of your brain. It grows out of the same tissue before you’re born and stays wired right into it, which makes the back of your eye the only bit of brain anyone can see without surgery. That’s what the eye doctor is looking at when they shine that little light in!
We started reading about eyes and couldn’t stop. Here are eight things that surprised us, all checked against the research, plus a couple of diagrams we drew to keep it straight.
1. It Can See a Single Photon
A photon is the smallest bit of light there is. In 2016, researchers in New York and Vienna built a light source that fired one photon at a time, sat people in the dark and asked them when they’d seen it. People got it right 51.6 percent of the time.
That’s barely better than guessing, but over more than 30,000 tries it held up. One photon is enough. And when a second one came along a few seconds later, people got even better at catching it, as if the first had woken something up. We love that.
About That Candle 30 Miles Away
You’ve probably heard you can see a candle flame 30 miles off on a dark night. Two Texas A&M astronomers checked in 2015, and the real number is closer to 1.6 miles. Still pretty good for a candle.
2. Your Cornea Breathes Air
The clear front of your eye has no blood vessels, since blood would get in the way of seeing. Instead it pulls most of its oxygen right out of the air, through the thin layer of tears you spread around every time you blink.
At night, with your eyes shut, it borrows from the blood vessels in your eyelids. That’s why eye doctors don’t love it when people sleep in their contacts.
3. About a Million Colors
Most people have three kinds of cones, the cells that see color. Your brain compares what each one picks up, and from that mix, the usual estimate is that you can tell around a million colors apart.
Some newer counts come in lower, and a lot depends on the light. Either way, that’s a whole lot of world from three kinds of cell. We went deep on just one color in our story on blue.
4. Your Sharp Vision Is Tiny
Hold your thumb out at arm’s length. Everything you’re seeing sharply right now is about twice that wide. The rest is blurrier than you’d guess. Your eyes just move fast enough that you never notice.
All that detail comes from the fovea, a little pit at the back of the eye about 1.5 millimeters across. It’s packed with cones, roughly 199,000 per square millimeter, and has no rods at all. Your whole retina holds around 92 million rods and 4.6 million cones. The fovea is less than one percent of it, but a huge share of your brain’s seeing goes to that one spot.
5. You Have a Blind Spot
Where the optic nerve leaves the back of each eye, there are no rods or cones at all. That’s a real hole in what you see. You never notice it because your other eye covers for it, and your brain fills the gap with whatever’s around it.
You can find yours right here.
6. You’re Blind Three Times a Second
Your eyes don’t sweep smoothly across a room. They jump, about three times a second, and while they’re moving, your brain mostly turns the picture off so you don’t see a blur.
Look in a mirror and glance from one eye to the other. You’ll never see your eyes move, even though anyone standing next to you can. Add up all those little jumps over a day and it comes to more than an hour your brain simply cuts out.
7. Your Two Irises Don’t Match
Eye color runs in families, but the little ridges and flecks in your iris form at random while you’re growing. A 2001 Cambridge study of 2.3 million iris pairs put the odds of two matching by chance at about one in 7 billion.
They’re so random that your left and right irises don’t match each other. Identical twins don’t match either, at least as far as an iris scanner can tell.
8. Every Blue Eye Shares One Ancestor
Way back, everyone had brown eyes. In 2008, researchers in Copenhagen traced blue eyes to one small genetic change that turns down the brown pigment in the iris. Blue-eyed people from Denmark to Jordan all carry the exact same change, which points to a single ancestor who lived somewhere between 6,000 and 10,000 years ago.
There’s also no blue pigment in a blue eye. With less pigment in the way, the iris scatters blue light back out, the same way the sky does.
One More, for Tonight
Next clear night, find a faint star and look right at it. Then look just a little to the side. It gets brighter! The cells that work best in the dark sit a bit off center, so dim things show up better out of the corner of your eye.
There’s lots more on how we take in the world over in The Senses, and our animal guide has a few eyes that make ours look ordinary.
Via Nature Communications, arXiv, Journal of Comparative Neurology, Proceedings of the Royal Society B, University of Copenhagen and Frontiers in Systems Neuroscience. Diagrams by Moss & Fog.
