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Connecting the physics of light to how the human eye processes images.
Did you know that right now, your eyes are actually seeing the world upside down, and your brain has to flip it over so you don't get dizzy?
To see anything, light must first enter your eye. It enters through a dark opening called the pupil. Have you ever noticed how your pupils look like black circles? That is actually a hole! The colored part of your eye, the iris, acts like a gatekeeper. It is a muscle that changes the size of the pupil. In bright light, the iris makes the pupil smaller to protect your eye. In the dark, it opens wide to let in as much light as possible. Light travels fast, at a speed of approximately kilometers per second, so your iris has to react quickly!
1. You walk from the bright sunny lobby into a dark theater. 2. Your iris senses the low light. 3. The iris muscles relax, making your pupil larger. 4. More light enters, allowing you to find your seat in the dark.
Quick Check
Which part of the eye is actually a muscle that changes the size of the pupil?
Answer
The iris.
Once light passes through the pupil, it hits the lens. The lens is clear and shaped like a bean. Its job is refraction, which means 'bending light.' Because the lens is curved, it bends the light rays inward so they point toward a single spot. This is just like a magnifying glass! The goal is to focus the light perfectly on the back of the eye. If the light doesn't bend at the right angle, the image will look blurry.
1. You hold a pencil close to your face. 2. Tiny muscles pull on your lens to change its shape. 3. The lens bends the light rays at a sharper angle to keep the pencil sharp. 4. The path of the light can be thought of as: .
Quick Check
What is the scientific term for when the lens bends light rays?
Answer
Refraction.
The back of your eye is covered in a layer called the retina. Think of the retina as a movie screen. When the lens focuses light onto the retina, it creates an image. However, because of the way the lens bends light, the image on the retina is actually upside down! To fix this, the retina sends electrical signals through the optic nerve to your brain. Your brain is the ultimate computer; it receives the signal and flips the image right-side up so you can understand what you are seeing.
Imagine you are looking at a meter tall tree. 1. Light reflects off the tree and enters your eye. 2. The lens refracts the light, projecting a tiny, upside-down tree onto your retina. 3. The optic nerve carries this 'upside-down' data to the brain. 4. The brain processes the data and tells you: 'That is a right-side up tree!'
What is the main job of the lens in the human eye?
Where is the image actually 'projected' inside the eye?
The image formed on the retina is naturally right-side up.
Review Tomorrow
Tomorrow, try to explain to a friend or family member why our pupils get bigger when the lights go out.
Practice Activity
Stand in front of a mirror in a dark room with a small flashlight. Shine the light near (not directly into!) your eye and watch how your iris quickly shrinks your pupil!