Cow's Eye Dissection
Watch: The steps of a cow's eye dissection
At the Exploratorium, we dissect cows’ eyes to show people how an eye works. This Web site shows photos and videos of a dissection. If you try this at home, wash your hands after the dissection. Wear latex gloves if you have cuts in your hands.
Learn: How does your eye work?
Experimenting with a Lens
To understand how your eye makes an image of the world, you need to know a little bit about lenses. Learn about lenses and experiment with a magnifying glass to discover how light makes an image of the world.
What's Going On?
Suppose you use your magnifying glass to make an image of a tree on a sunny day. You hold your lens between the tree and a piece of paper. You move the lens to just the right spot. Voilà! There’s an image of the tree. That image is made of light.
Sunlight bounces off the tree and spreads out in all directions. Your lens gathers the light shining out in all directions from each spot on that tree and bends that light so it all comes back together on a single spot on your piece of paper. So light shining from a leaf at the top of the tree ends up on one spot on your paper. Light shining from a spot on the tree’s trunk ends up in a different spot on your paper. All these spots of light blend together in your eye to make an image.
This works because the lens in your magnifying glass is carefully shaped to bend light in a particular, predictable way. The lens is shaped to bend light rays so that they come together and then spread apart to make an image.The lens of your magnifying glass is probably fat in the middle and thin at the edges. If you took the lens out of the magnifying glass, it would look like this image to the right. The surface of this lens is curved. It’s that curve that makes light bend when it shines through your lens.
Lenses and light
We took this picture to show you what happens when several beams of light encounter a lens.
When nothing gets in its way, light travels in a nice straight line. But when you put a chunk of clear glass or plastic in the way, light may not keep traveling straight. If light encounters a piece of glass or plastic at an angle, the light bends. The bigger the angle, the more the light bends. The light may bend again when it moves from inside the glass or plastic into the air. If the light meets the air at an angle, the light bends. The bigger the angle, the bigger the bend.
The beams of light shine through the clear plastic of the lens and encounter the curved surface where the plastic of the lens meets the air. Beams of light that meet this surface straight on keep on going straight. But some beams of light meet the curved surface at an angle. Those beams bend. This lens is curved on one side and flat on the other. The lens in your eye is curved on both sides, but it works in the same way.
Light and your eye
Once you know how lenses work, you can follow the path of light through your eye and find out how your eye uses light to make an image. How does light let you see these words? Here’s how:
- Suppose you’re looking out the window on a sunny day and you see a tree. You see that tree because light from the sun hit that tree. Some of that light reflected from the tree—it bounced off the tree like a ball bouncing off a wall.
- Some of that reflected light hit you right in the eye. That reflected light goes through the clear cornea of your eye. As it goes through the cornea, it bends a little.
- The light shines through your pupil, the dark hole in the middle of your eye.
- The light shines through the lens of your eye. The lens in your eye bends the light that has reflected from that tree to make a perfect little upside-down picture of the tree on the back of your eyeball. (Find out more about lenses and how bending light makes pictures.)
- At the back of your eyeball, there’s a layer of cells that are sensitive to light called the retina. When the picture of the tree shines on the retina, the light-sensitive cells send messages to your brain.
- Your brain takes the information from your retina and puts it together to make an image of the tree in your mind.
- Weird, isn’t it? You think you see the tree—but what you see is the light that bounced off the tree and got into your eye. Or if you really want to get picky, what you really see is the fixed-up picture that your brain makes up from the mixed signals it gets from your eye. Amazing!
Paul Doherty’s Eye Activities: Paul Doherty’s eye activities are part of the Exploratorium Teacher Institute’s month-long workshop for high school science teachers.
Pictures from Light: With a lens, you can bend light to make pictures of the world.
Human Body Explorations: This Exploratorium publication contains several eye-related activities, including Tunnel of Light, Something in Your Eye, and A Hole New Experience.
Eye Exhibits: The Exploratorium has developed over 50 exhibits relating to the eye and perception. Many of these can be found in our partner museums.
Pathway: Looking Without Seeing: Can you always trust your eyes? This Field Trip Pathway will test the limit of your visual perception.
Pathway: Eyeballs: Investigate how we see things using this Field Trip Pathway.
Not Fade Away: Joel Deutsch shares his journey into blindness as his retina slowly deteriorates due to retinitis pigmentosa .
Sighting the First Sense: This student-created Thinkquest site provides an interactive introduction to the eye, vision, and perception.
The Joy of Visual Perception: This online book presents copious information and illustrations on diverse topics relating to human vision and perception.
Cut Out Dissection!: This page from PETAkids presents ethical arguments against animal dissection.
Do your own dissection
Download step-by-step instructions below for doing your own cow’s eye dissection. Instructions include an eye diagram, a glossary, and color photos for each step.
Producer: Noah Wittman
Content & Writing: Pat Murphy
Design & Implementation: Yael Braha
Photography: Amy Snyder
Videographer: Elisabeth M. Spencer
Video Producer: Dia Felix
Explainer: Kathryn Fleming
Editorial: Martha Steele
Content Review: Rilla Chaney, Paul Doherty, Karen Kalumuck, Darlene Librero, Linda Shore