How the human eye works

DW (English)

Lens, pupil, iris, retina - they're words that most of us are familiar with. But how do these elements interact so that our brains can produce an image of the outside world?

Transcript

00:00 Our eyes are constantly active, exploring our surroundings and warning us of dangers.

00:08 They allow us to see a world that is full of shapes and colors.

00:13 But producing images that the brain can actually process is not so easy.

00:18 You need just the right amount of light.

00:21 So that we can recognize weak light sources at night and not be dazzled during the day, a muscle, the iris,

00:29 regulates how much light enters the eye.

00:32 When it contracts, the diameter of the pupils is just 2 mm.

00:36 When opened, it enlarges to 8 mm.

00:40 The color of the iris is determined by melanins, color pigments that also determine our skin color.

00:47 A darker color iris gives more protection from UV rays, making the eye less sensitive to light.

00:53 The next challenge is focusing.

00:56 The eye has a lens, a transparent ball of protein that focuses light rays and projects them onto the retina.

01:03 The ciliary muscles can stretch or curve the lens, allowing it to focus on different distances.

01:11 The more concentrated the rays are when they hit the retina, the sharper the image.

01:16 The yellow dot shows where the density of light-sensitive receptor cells is highest.

01:21 That's where our vision is sharpest.

01:24 But these receptors vary.

01:26 The rod cells are especially sensitive to light.

01:29 Then there are three different types of cone cells.

01:32 Some cones react to light in the blue spectrum, others in the green or red spectrum.

01:38 The optic nerve transmits impulses from over 100 million of these receptors to the brain.

01:44 When things move fast, it's too much for the brain.

01:47 This is why the eyes are good at rotating in all directions so they can track moving objects.

01:55 The brain receives images from both eyes at once.

01:58 Then it goes to work with the information.

02:02 The visual center at the back of the brain turns the countless pieces of information into a colorful three-dimensional world.

02:13 The lacrimal glands constantly lubricate our eyes with tear fluid so they can move easily.

02:19 They wash out foreign particles, too.

02:23 And our tears also allow the eyes to express our emotions.

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