3.40 Human Eye Facts & Anatomy

After light enters the human eye, it is eventually transmitted through signals to the brain. Learn about the similarities between a camera and the eye, and the six parts of the human eye.

The Camera

The eye is sometimes compared to a camera. Let’s examine this image of a camera:

There is a clear covering that allows light to pass through into the camera. The first part shown in the image is a brown membrane with a hole. The membrane is called a shutter, and the hole is the aperture. Do you think light can get through the shutter? Yes, it can but only through the aperture.

Next, there is the lens. The light reflecting off objects being photographed must pass through the aperture, then through the lens. The amount of light that passes through the aperture is controlled by the shutter opening wider or becoming narrower. If the shutter is closed, no light gets through to reach the film. When the shutter is open, the light passes through the lens. The lens points to or focuses the light on the film at the back of the camera.

Do you see how the light from the red arrow is being projected through the aperture, then through the lens and finally onto the film? Isn’t it interesting that the projected image is upside down on the film? When the film is developed you can look at the image right-side up.

1. Main points:

A camera works by allowing light to pass through an aperture in the shutter, then through a lens that focuses the light onto the film, creating an upside-down image that is later developed into a right-side up photograph.

2. Questions:

• Content Analysis: How does light travel through a camera to create an image?
• Contextual Analysis: Why is the image projected onto the film upside down?
• Linguistic Analysis: What words in the text describe the function of the camera’s parts?

3. Further Discussion:

How is the way a camera captures an image similar to or different from how our eyes see?

4. Answers:

• Content Analysis: "The light reflecting off objects being photographed must pass through the aperture, then through the lens… The lens points to or focuses the light on the film at the back of the camera."
• Contextual Analysis: "Isn’t it interesting that the projected image is upside down on the film?"
• Linguistic Analysis: Words like "aperture," "shutter," "lens," "focuses," and "projected" describe the function of the camera’s parts.

The Human Eye

Let’s take a look at the eye.

Let’s imagine that you see a clown. The light rays coming from the clown and everything around it pass through the clear membrane at the front of the eye. This clear membrane is called the cornea. Can you see the clown when the rays have only passed the cornea? No, the light rays must enter through the hole called the pupil.

The pupil is like the aperture in the camera. The pupil is in the muscular tissue called the iris. It’s like the camera shutter. When the iris contracts and relaxes, the pupil gets wider or narrower. The color of the iris provides your eye color. Now what happens when the iris contracts and the pupil is open? The light passes through the pupil. It then passes through the lens, which focuses the rays on the back of the eye called the retina.

The retina is like the film in the camera. It’s made up of special cells called rods and cones. These cells generate a signal. The signal travels through the optic nerve, a message pathway to the brain. In the brain the signal is interpreted.

Ah! Now it’s possible to see the clown!

1. Main points:

The human eye functions similarly to a camera, with the cornea allowing light in, the iris and pupil controlling light entry, the lens focusing light on the retina, and the retina’s rods and cones converting light into signals sent to the brain via the optic nerve for interpretation.

2. Questions:

• Content Analysis: How do the different parts of the eye work together to enable us to see?
• Contextual Analysis: Why is the pupil’s ability to change size important for vision?
• Linguistic Analysis: What words in the text describe the process of seeing through the eye?

3. Further Discussion:

How does the eye’s ability to adjust to different light conditions affect our daily activities?

4. Answers:

• Content Analysis: "The light rays… pass through the cornea… The pupil… in the iris… contracts and relaxes… The light passes through the lens, which focuses the rays on the retina."
• Contextual Analysis: "When the iris contracts and relaxes, the pupil gets wider or narrower," allowing the right amount of light to enter.
• Linguistic Analysis: Words like "focuses," "generates a signal," and "interpreted" describe the process of seeing through the eye.

Lesson Summary

There are six parts of the eye that you need to remember. Try to remember them in the order in which light enters the eye.

• The cornea is the clear layer over the eye.
• The iris is the muscular tissue that can contract or relax to control the size of the pupil.
• The pupil, or aperture, lets the light through to the lens.
• The lens focuses the light on the retina.
• The retina is located at the back of the eye. It contains the rods and cones, which are special cells. The retina is like the film in a camera.
• The optic nerve transports the signals from the retina to the brain.