00:00Okay, open your eyes, and let the world in.
00:05Follow me on a journey into your body's second most complex organ after the brain.
00:11But pay attention now! All of this will happen in the blink of an eye.
00:15First job at hand, get inside.
00:18Let's follow this dust particle.
00:20It floats up and lands on the cornea, that's the outer dome covering your eye.
00:25You can feel its shape right now.
00:26Close your lids, put your index finger over them, wash your hands first, and move your eyes side to side.
00:34Feel that bump? That's it.
00:37No blood vessels here. Your corneas get oxygen directly from the air, so your eyes breathe.
00:43But it's full of super-sensitive nerves.
00:46They sense something foreign, and the fastest muscles in your body get to work.
00:50Your eye blinks. That dust particle gets washed away.
00:54Let's try this again.
00:56The only way to get into the eye is with light.
00:59That's how your vision works, and it's why you can't see in the dark.
01:03Duh.
01:04But how do we see objects that don't emit light themselves?
01:08The light coming from the source, be it a light bulb or the sun, hits the object.
01:13Some of it gets absorbed, and some of it bounces off and enters our eye.
01:18It's a nice reflection on you.
01:20More on that a bit later.
01:22For now, it's way too dark in this room.
01:25Somebody flip a switch.
01:27Ah, that's better.
01:28We zoom along, going the speed of light, and pass through the cornean.
01:32It's dome-shaped to help focus light where it goes next, the lens.
01:36But before we get there, we pass through the pupil.
01:40It's not a black circle in your eyeball.
01:42It's a hole.
01:44Black because there's no light inside your eye.
01:47Red eye and flash photos is because the light from the camera is passing through and bouncing off the back of your eye.
01:53It's full of blood vessels in there, so your pupils glow red.
01:57But this pupil's too small, I can barely squeeze through.
02:01Ow, the light's too bright.
02:04The tiny muscles in your irises, that's the color part of your eye we know and admire, have relaxed to make the pupil smaller.
02:11This doesn't let too much light in and protects your eyes from excessive brightness.
02:16Turn the lights down a little, and the iris muscles contract.
02:20The pupil gets bigger as the eye demands more light to be let through.
02:24There we go.
02:26We pass through and come upon the aspirin-sized lens.
02:30Like your ears and nose, your lens continues to grow throughout your whole life.
02:35And just like in a camera, it focuses the light even more.
02:39That way, it hits exactly where it needs to be on the retina.
02:43That's the back of the eye.
02:45I can see our human here is nearsighted.
02:47Things far away are blurry.
02:50Look at where the lens is focusing the light into a single point.
02:53It's not on the retina, but a little before it.
02:56This happens when your eye is slightly too long.
02:59If you're farsighted, things up close are blurry, that focal point goes behind the eye because your eyeball's too short.
03:07Don't worry, it's nothing a pair of glasses can't fix.
03:10Ooh, it's roomy in here that I thought.
03:13Hello?
03:14Hello?
03:14Well, it's because you only see about one-sixth of your eye when you look in the mirror.
03:21The rest of it is inside your head.
03:23Your eyes themselves are about the size of ping-pong balls.
03:27And just within this seemingly small little organ, there are over two million working parts.
03:33Whoa, hold on.
03:34Everything shifting to the right.
03:36Whoa, now to the left.
03:38Our human must be looking for something.
03:40Your eyes move thanks to six muscles holding them in the socket.
03:44Certain ones contract, and your glance changes direction.
03:48Up, down, left, right, all around.
03:51Okay, things have calmed down, so let's continue following the light projection.
03:55Say, you're looking at a big, bright apple sitting on a blue table.
04:00A yellow light bulb hangs above.
04:02If you could see the image in here, it'd be upside down on the back screen of your eye.
04:07It's because the lens bent the beam.
04:10But you can't see this picture on the retina because eyes aren't projectors.
04:14It's just light hitting some tissue on the back wall of your eyeball.
04:18There are nerves and special receptors back there.
04:21They turn the light coming into your eye into nerve impulses the brain then decodes and makes sense of.
04:27So, we journey further.
04:30Here are those receptors.
04:31They can be cones or rods.
04:33Cones are the reason your incredible eye can detect up to seven million colors.
04:38Surprisingly, though, your cones come in only three types.
04:42Red, green, and blue.
04:44It's the combination of their work that allows you to see magenta, chartreuse, or cyan.
04:49Except for colorblind people.
04:51They might be missing one of those types, or they don't work as they should.
04:55So, these people don't see certain colors at all, or things aren't true to their color.
05:01For example, if there's something wrong with your green-sensitive cones, green and yellow look reddish-brown.
05:07There are also your rods.
05:09They pick up black, white, and over 500 shades of gray in between.
05:14Way more than 50 shades of gray.
05:16You also have more of them than cones, and the rods are mostly in charge of your peripherals.
05:22Yep, everything outside your direct field of vision looks like old black-and-white TV.
05:28Rods also help you see in low light.
05:31The photoreceptors in your eyes are so sensitive, they can even project an image when it's not even there.
05:37Ever looked at a bright light, closed your eyes, or turn off the light, and you can still see the shape of that light bulb floating before your eyes?
05:45That's your photoreceptors continuing to send visual information to your brain.
05:50Your rods and cones are connected to neurons, so the data gets past there and makes its way to your optic nerve.
05:57This is where blood vessels and the main path to the brain enter and exit the eyeball.
06:03This is also where your blind spot is.
06:06There are no rods or cones here.
06:08You can test it, too.
06:10Grab a piece of paper and a pen.
06:12I'll wait.
06:13Okay, make a dot on the left side and a plus about a hand's length to the right of it.
06:19Hold the paper at arm's length, close your right eye, and stare at the plus sign.
06:25The dot will disappear because it's in your blind spot.
06:29If it doesn't, move the paper closer or further away until the dot disappears.
06:35Do the same thing with your left eye closed and stare at the dot with your right eye.
06:39The plus sign will vanish when it enters your blind spot.
06:43You don't notice these spots because your brain fills in the missing information.
06:46So, we travel along the optic nerve and into the message decoder, your brain.
06:53Ow!
06:54Shocked me!
06:55Oh yeah, we're in the nervous system now.
06:58So, we're traveling with electric impulses.
07:01The optic nerve leads into your brain's visual cortex.
07:04This is where that upside-down image gets translated into something we understand.
07:09That's a blue table with a red apple sitting on it.
07:13A yellow light shines above.
07:14And your brain knows up and down thanks to your ears.
07:19Well, your balance system, which is mostly in your ears.
07:22Anyway.
07:23The brain is also where missing puzzle pieces, like the stuff in your blind spot, gets filled
07:28in with information based on its vast collection of archives.
07:32The dot on the left is my blind spot, so I'll just fill in the missing space and make it look
07:38like the paper continues seamlessly there.
07:41But our journey didn't start when we entered the eye with light.
07:44It started when light from a source bounced off an object and then into the eye.
07:50Why do we see different colors?
07:51Because visible light travels in different wavelengths.
07:55That red apple on the table absorbs other colors and reflects red-white waves.
08:00Also pronounced as red light waves.
08:02The table reflects blue and so on.
08:05Black objects absorb all the light, and white things reflect most of it.
08:10Those reflected waves hit cones and rods sensitive to them, and your world is filled with color.
08:16Especially red-white waves.
08:18You'll see.
08:18Black objects
08:20You
08:21You
08:22You
08:22You
08:22You
08:23You
08:24You
08:29You
08:31You
08:40You
08:42You
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