1Okay, open your eyes and let the world in.
2Follow me on a journey into your body's second most complex organ after the brain.
3But pay attention now, all of this will happen in the blink of an eye.
4First job at hand: get inside.
5Let's follow this dust particle.
6It floats up and lands on the cornea, that's the outer dome covering your eye.
7You can feel its shape right now.
8Close your lids, put your index finger over them (wash your hands first),
9and move your eyes side to side.
10Feel that bump? That's it.
11No blood vessels here.
12Your corneas get oxygen directly from the air,
13so your eyes "breathe."
14But it's full of super-sensitive nerves.
15They sense something foreign, and the fastest muscles in your body get to work.
16Your eye blinks, that dust particle gets washed away.
17Let's try this again.
18The only way to get into the eye is with light.
19That's how your vision works and it's why you can't see in the dark.
20Duh.
21But how do we see objects that don't emit light themselves?
22The light coming from the source, be it a light bulb or the Sun, hits the object.
23Some of it gets absorbed, and some of it bounces off and enters our eye.
24It's a nice reflection on you, more on that a bit later.
25For now, it's way too dark in this room.
26Somebody, flip a switch!
27Ah, that's better.
28We zoom along, going the speed of light, and pass through the cornea.
29Its dome shape to help focus light where it goes next: the lens.
30But before we get there, we pass through the pupil.
31It's not a black circle in your eyeball, it's a hole,
32black because there's no light inside your eye.
33Red-eye in flash photos is because the light from the camera is passing through and bouncing off the back of your eye.
34It's full of blood vessels in there, so your pupils glow red.
35But this pupil's too small, I can barely squeeze through!
36How? The light's too bright.
37The tiny muscles in your irises (that's the colored part of your eye we know and admire)
38have relaxed to make the pupil smaller.
39This doesn't let too much light in and protects your eyes from excessive brightness.
40Turn the lights down a little, and the iris muscles contract.
41The pupil gets bigger as the eye demands more light to be let through.
42There we go.
43We pass through and come upon the aspirin-sized lens.
44Like your ears and nose, your lens continues to grow throughout your whole life.
45And just like in a camera, it focuses the light even more,
46that way, it hits exactly where it needs to be on the retina,
47that's the back of the eye.
48I can see our human here is nearsighted.
49Things far away are blurry.
50Look at where the lens is focusing the light into a single point.
51It's not on the retina but a little before it.
52This happens when your eye is slightly too long.
53If you're farsighted, things up close are blurry.
54That focal point goes behind the eye because your eyeball's too short.
55Don't worry, it's nothing a pair of glasses can't fix.
56Oh, it's roomier in here than I thought.
57Hello!
58Well, it's because you only see about 1/6 of your eye when you look in the mirror
59the rest of it is inside your head.
60Your eyes themselves are about the size of ping-pong balls,
61and just within this seemingly small little organ,
62there are over 2 million working parts.
63Whoa, hold on, everything's shifting to the right.
64Whoa, now to the left!
65Our human must be looking for something.
66Your eyes move thanks to six muscles holding them in the socket.
67Certain ones contract, and your glance changes direction:
68up, down, left, right, all around.
69Okay, things have calmed down, so let's continue following the light projection.
70Say you're looking at a big, bright apple sitting on a blue table.
71A yellow light bulb hangs above.
72If you could see the image in here,
73it'd be upside down on the back screen of your eye.
74It's because the lens bent the beam.
75But you can't see this picture on the retina because eyes aren't projectors,
76it's just light hitting some tissue on the back wall of your eyeball.
77There are nerves and special receptors back there
78they turn the light coming into your eye into nerve impulses the brain then decodes and makes sense of.
79So, we journey further.
80Here are those receptors. They can be cones or rods.
81Cones are the reason your incredible eye can detect up to 7 million colors.
82Surprisingly, though, your cones come in only three types: red, green, and blue.
83It's the combination of their work that allows you to see magenta, chartreuse, or cyan.
84Except for colorblind people, they might be missing one of those types,
85or they don't work as they should.
86So, these people don't see certain colors at all,
87or things aren't true to their color.
88For example, if there's something wrong with your green-sensitive cones,
89green and yellow look reddish-brown.
90There are also your rods,
91they pick up black, white, and over 500 shades of gray in between
92You also have more of them than cones,
93and the rods are mostly in charge of your peripherals.
94Yep, everything outside your direct field of vision looks like an old black-and-white TV.
95Rods also help you see in low light.
96The photoreceptors in your eyes are so sensitive
97they can even project an image when it's not even there.
98Ever looked at a bright light, closed your eyes or turned off the light,
99and you can still see the shape of that light bulb floating before your eyes?
100That's your photoreceptors continuing to send visual information to your brain.
101Your rods and cones are connected to neurons,
102so the data gets passed there and makes its way to your optic nerve.
103This is where blood vessels and the main paths to the brain enter and exit the eyeball.
104This is also where your blind spot is, there are no rods or cones here.
105You can test it too. Grab a piece of paper and a pen.
106I'll wait... okay, make a dot on the left side
107and a plus about a hand's length to the right of it.
108Hold the paper at arm's length, close your right eye, and stare at the plus sign.
109The dot will disappear because it's in your blind spot.
110If it doesn't, move the paper closer or further away until the dot disappears.
111Do the same thing with your left eye closed, and stare at the dot with your right eye.
112The plus sign will vanish when it enters your blind spot.
113You don't notice these spots because your brain fills in the missing information.
114So, we travel along the optic nerve and into the message decoder: your brain.
115Ow, shocked me.
116Oh yeah, we're in the nervous system now, so we're traveling with electric impulses.
117The optic nerve leads into your brain's visual cortex.
118This is where that upside-down image gets translated into something we understand.
119That's a blue table with a red apple sitting on it.
120A yellow light shines above.
121And your brain knows up and down thanks to your ears.
122Well, your balance system, which is mostly in your ears.
123Anyway, the brain is also where missing puzzle pieces, like the stuff in your blind spot,
124gets filled in with information based on its vast collection of archives.
125"The dot on the left is my blind spot,"
126"so I'll just fill in the missing space and make it look like the paper continued seamlessly there."
127But our journey didn't start when we entered the eye with light,
128it started when light from a source bounced off an object and then into the eye.
129Why do we see different colors?
130Because visible light travels in different wavelengths.
131That red apple on the table absorbs other colors and reflects "red whitelaves".
132also pronounced as red light waves.
133The table reflects blue, and so on.
134Black objects absorb all the light, and white things reflect most of it.
135Those reflected waves hit cones and rods sensitive to them,
136and your world is filled with color, especially "red whitelaves".