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A look at how our eyes capture light and how the brain flips images right-side up.
Did you know that right now, your eyes are actually seeing the whole world upside down, and your brain is 'fixing' it for you?
To see anything, we first need light. Light bounces off objects—like a ball or a tree—and travels into your eye. The first place it hits is the pupil, which is the black circle in the center of your eye. The pupil is actually a hole! It acts like a gatekeeper. In bright sunlight, your pupil gets very small to let in just a little light. In a dark room, it grows wide to grab as much light as possible. Surrounding the pupil is the iris, the colorful part of your eye that uses muscles to change the pupil's size.
Quick Check
What happens to your pupil when you walk into a very bright room?
Answer
The pupil gets smaller to let in less light.
Once light passes through the pupil, it hits the lens. The lens is clear and shaped like a bean. Its job is to focus the light, just like a camera lens. It bends the light rays so they land perfectly on the back of the eye, called the retina. Because of the way the lens bends the light, the image that hits the retina is actually upside down! If you are looking at a foot tall tree, the lens projects a tiny, upside-down version of that tree onto your retina.
You can see how a lens works using a magnifying glass. 1. Hold a magnifying glass near a white piece of paper. 2. Point it toward a bright window. 3. Move the glass until you see a clear picture of the window on the paper. 4. Notice that the window on the paper is upside down! This is exactly what your eye's lens does.
Quick Check
Which part of the eye is responsible for focusing light onto the retina?
Answer
The lens.
The retina is covered in millions of tiny cells that catch light. These cells turn the light into electrical signals. These signals travel through the optic nerve, which is like a high-speed cable connecting your eye to your brain. When the brain receives the 'upside-down' message, it instantly flips the image right-side up. It also combines the pictures from both eyes so you can see how far away things are. This happens so fast that you never even notice the world was upside down to begin with!
Imagine you are playing catch. 1. The ball moves at a speed of . 2. Your eyes capture the light from the ball. 3. The optic nerve sends the signal to the brain. 4. The brain flips the image and calculates the distance in milliseconds. 5. Your brain tells your hand to move to the exact spot to catch it.
Scientists once wore special goggles that flipped the world upside down. 1. On Day 1, they couldn't even reach for a cup of water correctly. 2. By Day 8, their brains adapted and 'flipped' the world back to normal for them! 3. This shows that the brain, not the eye, is what truly 'sees' the world.
Which part of the eye is the 'hole' that lets light in?
What is the name of the 'cable' that sends signals to the brain?
The image that hits the retina is right-side up.
Review Tomorrow
Tomorrow morning, look in the mirror and see if you can identify your pupil and iris. Try to remember which one is the 'gatekeeper' and which one is the 'muscle'.
Practice Activity
Find a flashlight and a dark room. Watch your pupils in a mirror as you turn the light on and off to see them change size!