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Why Does The Sun Hurt My Eyes?

The questions kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places adults forget to explore. With that in mind, we’ve started a new series called Science Question From a Toddler, which will use kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. I want the toddlers in your life to be a part of it! Send me their science questions and they may serve as the inspiration for a column. And now, our toddler …

Q: Why does the sun hurt my eyes? — Gabby B., age 3-and-a-half

Have you ever had a muscle cramp, Gabby? Maybe you ran and ran and then your side hurt. Or you stretched your leg and got a charley horse. The pain you feel in your eyes when you look at the sun is similar — you’re doing more work with a muscle than that muscle is used to performing, and doing it faster. All of a sudden, there’s a lot more light trying to shove its way into your eye at once, and, in an effort at crowd control, your pupil contracts. The sun is “such an unusually powerful stimulus that the iris muscle is trying to get so small so quickly that it hurts,” Dr. Jeffrey Goldberg, chair of Stanford University’s department of ophthalmology, wrote to me in an email.

That might happen to you every day of the week, especially in summer when the sunlight is more intense. But there are other, rarer ways the sun can hurt your eyes. Like giving your eyes a sunburn. There’s a condition called photokeratitis (aka “snow blindness”) that really is similar to the burning, blistery rash that the sun can cause on skin. Just as in sunburn, ultraviolet radiation from the sun damages the top layer of cells — in this case, those of the cornea, the transparent barrier that covers the front of the eye. You’re most likely to get it when you’re looking at bright sunlight reflected off a surface like snow or water, but people also get photokeratitis in tanning beds or while welding. There’s even been a reported case of people getting photokeratitis from stage lights while partying in a nightclub. (Gabby, if you’re engaging in any of these activities, let’s talk.)

Glen Jeffery, professor of neuroscience at University College London, has had firsthand experience with photokeratitis from the time he spent studying animals that live in extreme visual environments, like the Arctic. There, creatures of all kinds have to adjust to long, bright days in summer and long, dark nights in winter. And that was where Jeffery noticed something odd. When he was out on the snow in bright sunlight, his eyes hurt. But “you’ll look down, and the husky next to you will be perfectly happy,” he told me. Humans who live in the Arctic, like the Inuit people, have developed special goggles carved from bone and wood that have a tiny slit or slits that let in only a small amount of light — enough to see, not enough to get burned. But the sun doesn’t seem to affect animals such as reindeer, polar bears and dogs the same way.

What gives? The secret is in the type of light and what our eyes do with it.

All light is actually electromagnetic radiation. It is tiny particles oscillating up and down in waves. We see those waves as colors. Different wavelengths show up for us as green, yellow, blue — all the colors of the rainbow. There are also wavelengths we can’t see, because our eyes block them. Infrared radiation has wavelengths that are too long to be visible to us. Ultraviolet radiation has wavelengths that are too short. They hit the lens of our eye, but they aren’t let through.

But when the lens blocks UV radiation, that energy has to go somewhere. Just like when a running back slams into a linebacker, both players aren’t able to stay still. Instead of passing through the cells that make up the lens of the eye, the oscillating particles crash into them, damaging the cells, and causing the pain of photokeratitis, Jeffery told me. Animals that can see UV light, on the other hand, whose lenses let the UV light pass, don’t have the same kind of problems. So, to him, the fact that reindeer and huskies didn’t seem to be getting photokeratitis was a sign that maybe they could see UV.

That was a pretty crazy idea. Scientists have long thought that mammals, as a whole, can’t see this stuff. It’s only in the last 15 years or so that anyone has questioned that truism. The re-evaluation started with rodents, which turned out to have light-sensing cells in their eyes that could see light with a wavelength of 365 nanometers — UV light is defined as light with a wavelength between about 400 nanometers and 10 nanometers. Light is visible to humans when the wavelengths are 400 to 700 nanometers.

Other mammals didn’t have those cells, though, so Jeffery tried another tack. He contacted a bunch of zoos and asked them to let him know anytime they had a mammal that died. When they did, he requested that the zoo extract an eyeball, freeze it and send it to him. (Scientists get very interesting mail.) He ended up with eyeballs from 38 species. “We put them into a machine that fired light through the lens and cornea. It started at 800 nanometers and worked down to 300, and we could see the point when it was blocked,” he told me. Primates never got past 395 nanometers, he said. But other mammals — including seals, bears and reindeer — could see much shorter wavelengths. Dogs could see down to 335 nanometers. Jeffery now thinks humans, and other primates, are the odd ones out.

“So to some extent, we need to turn your question right around,” Jeffery said. I told you the physical reasons why the sun hurts human eyes, but I can’t tell you why it hurts our eyes when it doesn’t seem to hurt a lot of other mammals. Jeffery speculated that primates ended up different because we have sharper vision than those other mammals. “All the primates, they pick stuff up, hold it in front of their face and have very high acuity. They see in incredible detail,” he said. Shorter wavelengths of light get in the way of that because they have more of a tendency to scatter, like light shimmering off a prism. Jeffery thinks our eyes evolved to block UV light because it got in the way of our detail vision. If he’s right, and nobody knows whether he is, it would mean that the sun hurts your eyes because you can see an ant on the end of a stick.

Gabby, there’s one last thing that I stumbled across that was too amazing not to share: The sun can also hurt your eyes even when you don’t feel any pain. There’s a disorder called solar retinopathy that kills cells in the retina (the light-sensitive tissue on the inside of the back of your eyeball) if you look directly at the sun for too long. It often doesn’t hurt, because the retina doesn’t have many pain receptors.

One of the ways we know: In 1993, researchers found an old man who had cancer in his right eye. He was going to lose that eye anyway, so he agreed to look at the sun for 10 straight minutes. Six days later, after his eyeball was removed during cancer surgery, the researchers dissected it and got a close-up look at the havoc. His eye cells were in serious trouble, beyond the damage from cancer. His mitochondria — structures within the cell that help generate energy and keep the cell alive — were swollen and bloated. The nuclei — the parts of the cells that store all-important DNA information — were shrinking, on the verge of breaking apart. Light from the sun had triggered chemical reactions that were killing the old man’s eye cells. If people look at the sun for only short periods, these effects can be reversible, but not always. And it can take a long time to heal. In the meantime, victims have to deal with blind spots, light sensitivity, color distortions and other vision problems (provided they plan on keeping their eyeballs).

So, my final advice for you, Gabby: Don’t stare directly at the sun, and read the fine print on any scientific study you volunteer for.

Read more science questions from a toddler: Why Am I Right-Handed?

Have your own science question from a toddler that you want answered? Send it in!

Maggie Koerth was a senior reporter for FiveThirtyEight.