Why do we have two eyes if we only see one image?
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This is But Why, a podcast for curious kids from Vermont Public. I'm Jane Lindholm. On this show, we take questions from curious kids all over the world and find interesting people to answer them. Many, many years ago now, we did a couple of episodes about our bodies and how they work. We had so many questions just about eyes that we actually didn't include eyes at all in those episodes. And we promised at the time we'd make another one just about eyes.

Well, time got away from us. But here we are now with not one, but two episodes about eyes, eyeballs, and eyeglasses. We decided to take your eyeball and sight questions with us to a doctor's office at the University of Vermont Medical Center, where I thought we could also find out what happens when you get an eye exam. I'm Sujata Singh. I'm the pediatric ophthalmologist at University of Vermont. I am a pediatric ophthalmologist.

do eye exams just for kids and also for older adults who have maybe trouble communicating. I also help develop vision for little kids because they're still learning how to see and that requires a whole different specialized skills compared to what the adult ophthalmologists have to be skilled in doing. What does the word ophthalmology mean? That's such a big and cool word, but you said you're an eye doctor. Why do we have to call you an ophthalmologist?

Because there are a few words that describe different kinds of eye doctors. An ophthalmologist, which has a few too many H's, I think, for most people, they are eye doctors and surgeons. So we went to medical school, did four years of training after medical school, and then

and learned how to take care of patients, take care of the surgeries, and I did an extra year to learn how to take care of kids, and so that's what an ophthalmologist does. Sometimes going to any doctor or any medical office can be really nerve-wracking and can make people anxious, or sometimes it's fun because you get to learn things about your body, and sometimes it's both.

Will you walk me through an eye exam while I ask you questions that kids have sent us about eyes? Yes, absolutely. Okay, what do I have to do first? We have you sit in the big chair. If you're really little, we'll have you sit with whoever brought you in on their lap because we want you to be comfortable and also tall enough for me to take a look at your eyes. All right, I'm going to sit in the chair. Melody, I think I'm big enough that I don't need to sit on your lap, so I'm going to sit by myself.

This chair kind of reminds me of a little bit like the chair you'd sit at at a dentist's office. It's pretty similar, I would say, but the best part about it is that it does this.

Ooh, I'm going back. Oh, I'm leaning back. It's time for a nap. It's a great spot for a nap, and then we can bring you back up with some buttons. And the kids are also usually clever, and they find it before I can show them. Okay, so you're sitting in front of me. What are you going to do to my eyes? Do I need to take off my glasses? So, yes, if you're wearing glasses, the first thing we'll do is look at you with your glasses on. One of the biggest things that...

help us decide how we can help you is getting your visual acuities. And so that's a fancy word for just knowing how you see. And we do both eyes, if you have two eyes. And we do one eye at a time usually. And so we use this guy.

which is called an occluder. It looks like a big spoon with a red see-through thing on the end. Yeah, it's like a soup spoon. We've had kids try to eat with the soup spoon. But we'll hold that right up to one of your eyes and have you look all the way down this very long room at the TV that has some either letters or we have all sorts of different symbols, also pictures, and we have...

the HOTV letters and then the alphabet also. So if I don't know my alphabet or don't use these types of letters, I can still do pictures. Yeah, absolutely. And if you're really, really shy or if you're still working on learning your letters, which is totally understandable, we also have these matching cards. So you might kind of recognize the HOTV, but it can be

You know, like I said, you can be either shy or not sure or have a lot of anxiety about being completely right. And so, but you are really good at matching, which most kids are. So we're talking about eyes and talking about eyesight. And so I think all of us probably already know that your eyes are the organs that help you see, right?

eyes made out of? Hi, my name is Rose and I live in Kitchener, Ontario, Canada. I am six and a half years old and my question is what are eyes made of? That is a really great question actually. I have to think about how to answer that. So your eyes are made out of what's called tissue. Different parts of the body is made out of different kinds of tissue, right? Like your bones aren't the same

thing as your skin, but they're both very important and they are built to give you the best function. And so they're made of, I don't know how exactly precise, how much to go into it, but there are very, it is a complex device. So there are a lot of different layers to it and they're highly, highly

highly specialized and very delicate. Eyes are complicated. It's no wonder Dr. Singh was trying to think hard about how to explain it all.

We'll get deeper into some of the structures of the eye in a minute, but just to give you a sense of what your eye is made of, there are blood vessels, a mucous membrane, nerve tissues, light receptors, muscles, and a see-through jelly-like substance that fills a lot of the inside of your eyeball called the vitreous humor. My name is Grace, and I live in Bend, Oregon, and I'm four years old. And my question is, how do eyes work?

Hi, my name is Iris. I'm seven years old. I live in the UK and my question is how do eyes work? Hello, my name is Ethan. I'm five years old. I live in St. Louis, California and my question is how do eyeballs see?

My name is Louis. I'm six years old. I'm in Varsanar in the Netherlands. My question is, how do your eyes work? I am Liam. I am four years old. I would like to know how eyeballs work.

The eyes, their job is to deliver images from the world into the brain. And so they're literally what we call optical devices. Glasses are optical devices. They're just there to deliver images. The front part of the eye is called the cornea. That's the clear cap of tissue that covers the colored part of the eye. The colored part of the eye, whether it's green or brown or blue or all sorts of different colors, is called the iris. It's like a ring of tissue with a hole in the center.

The black part is actually the hole in the center, and that's called the pupil, and it can change sizes. That's why we give you drops to make it as big as possible. Right behind that is called a lens. That's a disc-shaped structure. That can actually change size and give you different focuses. So you can see clearly in the distance or see clearly up close, but you can't see clearly at both spots at the same time. And that's because your lens can change shape.

and then the inside is filled with the jelly which is the funnest part of course and the back layer inside layer of the eye is the retina which is very complex and that's where you have your color vision that's where you have all the lights that fill up the world gets sharpened and focused into an actual image that's upside down and reversed when it lands similar to a camera it's it's

People describe it as the film of the eye. And then that gets all transmitted to your optic nerve, which is a cord that connects to the brain. And then the brain is actually where we see. So that's why I say that eyes are optical devices, whereas the brain is the actual, the seeing part of the body.

Yeah. If I could pop my eye out right now, which I won't try to do, could I, would it, if I tried to squeeze it, would it feel like a marble or would it squeeze like a tomato? Oh gosh. A lot of the time we call it the feeling of a grape.

So it's a little bit tense, but not that tense. So it is squeezable and poppable, but yes, thank you so much for not attempting that. It's highly not advisable. We're very big on eye protection and especially Pete's ophthalmology, so thank you. Hello, my name is Gideon. I'm nine years old. I live in Linden, Washington, and my question is, what is the cornea of your eyeball made of?

It's a thing called collagen. And the way that it's actually layered is it's shaped and stacked so that it's transparent, which is one of the most interesting things about it. In addition to how it curves is how much of a power you have in the eye. It's called a refractive power. And basically that's

That's kind of your glasses prescription that's built into the eye. So it's the shape of your eye, the shape of the cornea especially, that gives you that. The other thing that it does is keeps your eye comfortable. So it's supposed to be smooth and well hydrated, like wet, to get the best images to the back of your eye and also to feel comfortable. Hi, my name's Lane. I'm nine years old.

I'm from Vancouver, BC, Canada. And my question is, are your eyeballs really in the shape of balls? It's actually not. That's a really great question, actually, because it has different curvatures. So as you go into the cornea, it's a different kind of arc to it compared to the rest of the eyeball, the white part of the eye. So it's not quite all a ball, but eyeballs is still a really good word for it.

Hi, my name is Saul. I'm four years old. I'm from Brooklyn. And my question is, why do we have to blink? My name is Owen, and I'm six years old. And I live in Toronto, Ontario. And my question is, why do people blink?

Hi, my name is Jason. I live in Stanford, Connecticut and I'm five years old, five and a half years old. And my question is why do your eyes blink? Hi, my name is Logan. I'm five years old.

I live on London, Ontario. My question is how and why do you blink so fast? Hi, my name is Ananya. I am eight years old. I live in Cupertino, California. And my question is why do we blink?

Hi, I'm Chelsea. I'm eight years old and I'm from Taipei. My question is, why do we blink? My name is Cooper. I'm from Southern Maryland. I'm five years old and my question is, why do we blink our eyes?

Phoenix is five and lives in Seattle. He also wants to know why people blink. We blink to clear up our images and then also to hydrate our eye, to make it as not dry as possible and to make it feel smooth. And those are the main reasons. And also to blink out anything that might have any dust or anything. Sometimes lashes fall in there. So that's why we blink.

and to give it some nutrients also. Blinking is also a little bit like breathing, right? In that we don't, we have to do it, but we don't have to think about it. It happens automatically. Yes, exactly. Yeah, we don't have to think. A lot of kids want to know why we have tears. Hi, my name is Lucy. I'm six years old. I come from Seattle, Washington. And my question is, why do tears feel warm on your skin and where do they come from?

My name is Millie. I'm seven years old. I'm from British Columbia, Canada. And my question is, why does water come out of your eyes when you cry? My name is Otter. I'm five years old. I live in Waterville. My question is, where do your tears come from? Hi, my name is Eleanor. I'm five and a half years old. I'm from Chicago, Illinois.

My question is, where do tears come from? Hello, my name is Imogen. I'm from Australia and my question is how are teardrops made? Hello, my name is Ella. I live in Montreal, Canada and I'm from Japan and I'm four and a half years old.

And I want to know how your body makes tears. Hi, my name is Oliver. I'm six years old. I live in Miami, Florida, and I want to know how your eyes create tears. We also got this same question from Lillian. How do eyes make water to cry?

So the point of having tears is to keep that cornea happy and healthy and as smooth as possible. When it's like that, it allows the light rays that are bouncing around in the world to go into your eyeball as smoothly as possible, as sharply as possible. So that's one big thing. And it actually, you have to know some physical optics.

but the way the light rays go in, and then it bends through the cornea and actually the tears to get sharpened inside your eye. And you can see how light bends through different medium, like if you've ever put a straw in a glass.

Sorry, a glass of water. It doesn't stay straight through the glass of water, right? It bends. Or it looks like it's bending. It looks like it's bending, right, exactly. And that's because the light rays are traveling at different speed because it got bent as it's trying to pass through the water medium. And the same thing happens in your eyeball. And that's why the eyes are an optical device, because it changes the shape of the light rays that are going through it. And so that's a big part of what tears help with. And it also just helps, you know, kind of give nutrients and...

immune defenses and all sorts of things like that. Helps me blink my eyes and move my eyelids up and down smoothly. And also when I get a piece of dust in my eye, sometimes I make more tears. So it can also kind of help get things that aren't supposed to be in your eye out of your eye too, right? Like you can produce more tears to get bad stuff out of your eyes. Yeah, exactly. The eyes, the corneas actually have that almost the densest

amount of nerves in the whole body. It's a very tiny spot of tissue, but it has a lot of nerves. So that means it's really sensitive to foreign objects. And then when it feels that, all of this tearing happens. We call it reflexive tearing because you don't try to do it. You just do it automatically and then the tears wash it out. So yes, tears are our friend. My name is Khairan. I'm eight years old. I'm from Kigali, Rwanda.

My question is, why are our tears salty? Thank you. My name is Abby.

And I'm four years old. I live in Chicago, Illinois. And my question is, why do tears taste salty? Your tears taste salty because they are salty. You have salt in your blood, sweat, snot, and yes, tears. You need some salt to survive, and it's all throughout your body.

Your tears are made up of water, electrolytes, that's what tastes salty, proteins, oils, and sometimes hormones. And there are different kinds of tears. Basal tears are the kind that are constantly flowing across your eye to keep it lubricated, what Dr. Singh was talking about. Reflex tears are made when you've got that speck of dust or an eyelash in your eye that you need to get out. And emotional tears are the kind your body makes when you're sad or injured or sometimes when you're really happy.

Tears start toward the top of your eye and drain into ducts that go into your nose. That's why you might get a stuffy nose when you've been crying. Let's get back to the eye exam. Here's what comes next.

So if you're a new patient, then next I want to make sure that your pupils, that hole in the center of your eye, we want to make sure that that's normal. So we check that by using this light. I'll take off my glasses. And we make sure that your pupils are as big as possible. So the way that they get as big as possible is when the lights are down and you're looking way down to the other end of the room.

So then your eyes are really big and then I turn on my light and I get really close and we shine the light in one side and I see how well it reacts, meaning how well it gets smaller and then the other side. And then we keep doing that to make sure that it's normal and

How do I look? You look great. Very nice, brisk reaction. Your pupils are round. Okay, so you've shined some lights in my eyes. Now what do we do? So now we make sure that your visual fields, meaning your peripheral field, is normal. And peripheral means on the outside. So you're checking to make sure that not only can I see what's straight ahead of me, but I can also kind of see what's out of the corners of my eyes without having to move my eyes and move my head to the center. Yep, exactly. And so...

We want to make sure you can see as wide out as possible. And so usually with adults, you can have them follow directions. So you can have them cover up one eye and kind of put your fingers kind of out in that peripheral field and see whether they can see it while they're still looking at your nose. It's a funny exam to look at. But with kids, you know, they sometimes can't follow those directions, but they want to do a really good job. So we actually have some...

help, which is these are my medical devices. So I have some little toys, finger puppets usually work pretty well. They're the right size and they're bright. And I just have them wait and have them give me a nonverbal cue. So usually they...

I don't know about you, though. So you want me to tell you that I am seeing this other finger puppet that's not at your nose moving? Exactly. And so kids, little kids, will just automatically look for the motion and tell me that they can see it. So the one disadvantage of this is that I can't test one eye at a time, but I can get a kind of sense of what their peripheral visual fields are doing. Coming up, why is it that we have two eyes but only see one thing at a time?

Why can't our eyes move independently of one another? This is But Why, a podcast for curious kids. We're learning all about eyes with Dr. Sujata Singh. She's a pediatric ophthalmologist at the University of Vermont Medical Center. My name is Ashton. I live in Bochertone, Florida. I'm six years old. My question is, why do eyes look one way but not two?

Hi, my name is Eleanor. I'm from Buffalo, New York. My question is, why do you have two eyes but see one picture? My name is Rosie, and I am eight years old, and I live in Morristown, New Jersey. And my question is, if you only have two eyes, why do you see one picture? We have two eyes, but unlike some animals, our eyes don't move independently of one another, usually. So we're usually using...

two eyes to see the same thing. So why do we need two eyes if they're going to move in the same direction at the same time and see the same thing? You do have two eyes, and if you look at something, close one eye and then switch and close the other eye, you'll notice that the eye kind of, the object actually changes. So you can't see the same image

from both eyes, it's impossible because they are set in a different spot in space and you're not a cyclops, right? Both eyes aren't sitting in the same spot so they can have the same edge. They're coming at whatever you're looking at from different angles.

You can kind of test that if you put your finger in front of your eyes and in front of the thing that you're trying to look at and then close and open your different eyes and watch your finger move in front of you. Yeah, and the finger jumps around because that's the angle that your eye is looking at. And so what that tells you is that each eye is actually bringing a different image to the brain. And so that's what I was talking about earlier where those images, when you push them together, then you get...

3D vision, basically. So if you walk around, I mean, in a safe environment, if you walk around with one eye closed, you'll notice that you start to have trouble doing a few things. If you try to thread a needle, playing with blocks and stuff, you'll notice like all of a sudden you can't do it as easily or confidently as you could with two eyes. And that's what

the depth perception is giving you. So that's your highest level of vision. We call it binocular vision, meaning using two eyes. And you don't need it, you know, to live life, right? Like we have lots of doctors, teachers, and artists out there who can see really well out of one eye, not so well out of the other eye.

That's why we get two eyes, and it's nice to have a backup. One of the things Dr. Singh can test for is to make sure your eyes are aligned, meaning they move in the same direction at the same time.

Sometimes, and this is more common in kids, your eye or eyes might turn inward or outward. This condition is called strabismus. An ophthalmologist like Dr. Singh can work with you to correct strabismus with various treatments that include special glasses, medicine, and eye exercises. So the next thing is actually, since we were kind of talking about it already, let's talk about dentistry.

depth perception. So it's called, the fancy word is stereopsis. So stereo, like 3D kind of thing, and opsis meaning vision. And so I test your stereopsis with 3D images. I'm holding a little kind of booklet with, one has some shapes and numbers and animals, and then one has a fly on it. Yeah.

It has a very fun fly. And you can only see it with what I call the magic glasses. They look kind of like sunglasses, but really what they are is they have a filter on one side, filter on the other side. I have you put those on and then take a look. Whoa, they're 3D. That fly looks like its wings are coming out at me. Yeah. And so even a one-year-old might...

put on the glasses and actually automatically go after it. They might not say those words, the fly is coming out at me. I don't want it to jump off the page, but they'll automatically try to pick it up because that's what it looks like to them because they have 3D vision. And so basically this picture of a fly is actually two different pictures that are just slightly off.

And then your glasses have a filter in one eye and a filter in the other that are opposite to each other. So the pictures of the fly actually go into each eye differently, and then your brain has to push them together, and it pulls out this 3D image. And so that's a very, like, gross, meaning very not just gross because it's a fly, but also gross because it's a very large amount. So you can kind of tell it's popping out even when you're not wearing the glasses. Right.

But then we get into finer and finer levels of depth perception. Hi, I'm curious why our pupils get smaller. That black part of our eye in the center, sometimes it's a really big black circle, and sometimes if it's really bright outside, it can be a really small black circle. Yeah, no, that's a really good description of it. It's a really important part of our ability to see is how much light goes into our eyeball.

So if you, you can imagine if you have too much light going into your eyeball, you can't make out the objects that you want to look at with high definition and high level of details. So your pupils are what control the amount of light that goes in. So it's the nerves that come from your eyeball into your brain and then from your brain into your eyeball, back into your eyeball is the pupillary reflex.

and that's what controls what size it is and it's very, very sensitive to one, how bright it is out there and then also what part of the world you're focusing on, whether it's distance or near. So actually if you look up close, you'll notice that your pupils actually get smaller and also when you look in the distance, your pupils get bigger.

Sometimes eye doctors do a test where they put some drops into your eyes and it makes your pupils get bigger and then everything looks kind of fuzzy for a little while. What is that test about? Yeah, so it's actually a difference. It's a slight difference between kids and also adults for why we dilate. And it's also one of the most anxiety-provoking things that a pediatric ophthalmologist does. We

For the doctor or for the kid? For a doctor who's just learning and also for the kids. Because the kids will come in and they're wonderful about being appreciative of coming in, but they will talk about the drops all year long. So what is the dilation about? So we call it dilation, but it's actually two parts. One is dilation and the other word is cycloplegia, which is a big fancy word, which means that we make it so it's fuzzy. You can't focus up close.

So that's what those medications that are in the drops do. They make your pupil really, really big, just like when it's really dark. And the fancy word for that is dilation. And then we make it so that you can't focus up close, and the fancy word for that is cycloplegia.

And the reason why we do both those things is one, I can catch the structures that are in the back of the eye when the pupil is larger. So that's one helpful thing. And then the part where we make it so you can't focus means that I can measure your glasses prescription without you neutralizing my measurements because you're not able to focus. And that's what I do after dilation. I can show you if you'd like.

I check your glasses prescription with what's called a retinoscope. And it's this handheld kind of stick-looking instrument with a square top. And basically, it creates a rectangular light when we turn it on. And I can rotate the directions that the beam is pointing.

looking at and I hold it up to my eye with where the pupil is and then I shine it in your pupil. It's very bright. Yes it is and so I can see that actually you can kind of guess your glasses prescription you're a little bit nearsighted with some astigmatisms. Yes I am well done. Thank you. I was really impressed that Dr. Singh could look into my eyes and know how my eyes function.

I kind of wish I could see into my own eyes the same way she could see into them. Wouldn't it be cool to be able to look into your eyeball? I'm nearsighted, meaning I have trouble seeing things clearly that are far away. And I have some astigmatisms, which is when your lens or cornea isn't shaped quite right, leading to blurry vision. Because of those things, I need glasses. Lots of people wear glasses or contact lenses to help them see better. They are amazing inventions.

And we're going to learn more about how glasses help people see in our next episode, where we continue to hear about our amazing eyes and how they function. Dr. Sujata Singh from the University of Vermont Medical Center will be back with us, too, answering all of your eye-mazing questions. If you have a question about anything, have your adult send us a voice recording of you asking it. You can do it on a smartphone using a voice recorder or voice memo app.

Be sure to include your first name, where you live, and how old you are, and then your adult can email the file to questions at butwhykids.org. Or you can find out all of this information and submit your question directly at our website, butwhykids.org. But Why is produced at Vermont Public and distributed by PRX. Our team includes Melody Beaudet, Kiana Haskin, and me, Jane Lindholm.

Our theme music is by Luke Reynolds. We'll be back in two weeks with an all-new episode. Until then, stay curious. From PR.