How Do Your Eyes See?眼睛怎麼看見東西？

🔊Look up from this page at something far away, then look back down. Your eyes just changed their focus twice, and you probably did not even notice. Each eye is a ball about 2.4 centimeters across, filled with a clear jelly. Light bounces off everything around you, and some of it enters your eyes. Inside, the light is bent to make a tiny, sharp picture at the back of each eye, a lot like the picture inside a camera. Your eyes collect the light, but the seeing itself happens in your brain.

🔊The front of the eye lets light in. First comes the cornea, a clear, curved window that does most of the bending of light. Behind it is the iris, the colored part of your eye, which is a ring of muscle. The black circle in the middle of the iris is the pupil, and it is simply a hole. In bright sunlight, the iris makes the pupil as small as 2 millimeters to keep out extra light. In the dark, it opens the pupil to about 8 millimeters to catch as much light as it can. When you walk into a dark movie theater, your pupils open wide in seconds, but your eyes need several more minutes to get used to the dark.

🔊Behind the pupil is the lens, which fine-tunes the focus. A ring of tiny muscles around it changes its shape. To see something far away, the lens is pulled flatter. To see something up close, the muscles tighten and the lens gets rounder, so it bends light more. Light from each point of an object comes together at one point on the retina, the thin layer at the back of the eye. But here is a surprise: the picture on your retina is upside down and flipped left to right. Your brain sorts this out, so the world looks right side up.

🔊The retina is where light turns into nerve signals. It has more than 120 million light-sensing cells of two kinds. Rods work in dim light but cannot see color, which is why things look gray at night. Cones need brighter light, and they let you see colors and fine details. The signals leave the eye through the optic nerve, a cable of about a million nerve fibers. Where the optic nerve leaves the eye, there are no light-sensing cells at all, so every eye has a blind spot. You never notice it, because your two eyes cover for each other and your brain fills in the gap.

🔊Some eyes have trouble focusing. In a nearsighted eye, the eyeball is a little too long. Light from faraway things comes to a point in front of the retina, so the board at school looks blurry, but a book up close looks sharp. In a farsighted eye, the eyeball is a little too short, so things up close are harder to see clearly. Glasses fix both problems by bending the light a little before it enters the eye. Lenses for nearsighted eyes are thinner in the middle and spread the light out. Lenses for farsighted eyes are thicker in the middle and bring the light together.

🔊Taiwan has one of the highest rates of nearsightedness in the world, and most students here are nearsighted by the end of elementary school. Scientists have found that children who spend more time outdoors are less likely to become nearsighted, which is why Taiwan encourages two hours outside every day. When you read or use a screen, keep it at least 30 centimeters away, and every 30 minutes, rest your eyes for 10 minutes by looking far away. Never look straight at the Sun. Now try the blind spot test below the model and find the hole in your own vision.