Dear Prisha,
I got glasses as a kitten. I put on my new spectacles and was floored by the crisp details. I had no idea you could see individual leaves on trees. I thought everyone saw a green blur.
I talked about that with my friend Dr. Rachna Narula. She’s an eye doctor—called an optometrist—at Washington State University.
She told me that glasses help direct light to the correct spot in the eye so we can see.
“If someone’s eyeball is too short or too long, glasses help focus the image on the retina because that’s where the person can see,” Narula said.
We see because light gets inside our eyeballs and hits a layer of tissue at the very back called the retina. That tissue is full of cells that change light into electrical signals and shoot them up the optic nerve to our brains. Our brains turn the signals into images.
Getting light to hit our retinas depends on different parts of the eye working together. First, the light goes through the clear, squishy tissue at the front of the eye called the cornea.
The cornea bends the light to focus it toward the retina. Then, the lens, which is behind the cornea, bends the light a bit more.
Ideally, the cornea and lens work together to bend the light so it hits the retina.
But sometimes the distance from the cornea to the retina is too long or too short. The bent light hits in front of the retina or behind it. That makes our vision blurry.
Some people are near-sighted, or myopic. Their distance vision is blurry. Their eyeballs are too long, so the light focuses in front of the retina.
Some people are far-sighted, or hyperopic. Their up-close vision is blurry. Their eyeballs are too short, so the light focuses behind the retina.
Some people have astigmatism. Their cornea or lens isn’t perfectly round. It’s shaped more like an egg. That scatters the light, making it hit lots of places at the same time. That also makes things blurry.
An optometrist can measure the length and shape of our eyeballs. They use lots of instruments and ask us questions to figure out the perfect lens to correct our vision. Glasses bend the light before it gets to the cornea. So, the glasses lens, the cornea and the eye’s lens work together to bend the light just right.
A glasses lens can curve inward—called concave—to fix near-sightedness. Or the lens can curve outward—called convex—to fix far-sightedness.
You can tell what kind of lenses you have by looking closely at them.
“Concave lenses are thinnest at the center, and as you go out, they become thicker,” Narula said. “Convex lenses are the opposite. They’re thickest in the center and become thinner toward the edges.”
Some eyeballs need both kinds of correction, so some lenses do both.
Narula told me that lots of people don’t realize they’re having trouble seeing—like me thinking trees are green blobs. It can make doing stuff or learning difficult. It can give us headaches or make our eyes super tired.
That’s why we visit the optometrist. The right glasses can reframe how we see the world.
Sincerely,
Dr. Universe