Extraordinary eyes: take a look at how other animals see the world.
That's because not all eyes are the same. Throughout the animal kingdom, some species see the world in ways that humans can only imagine.
Light Into Sight
For all animals, seeing requires light, says eye doctor Robert Maloney. The simplest eyes are nothing more than collections of cells that sense light.
In humans, the process is more complicated (read How Does the Human Eye See? page 6). Light passes into the eye through a disc-shaped structure called a lens. It focuses the light onto the retina at the back of the eye. The retina is full of cells, called rods and cones, that sense light. Rods pick up dim light. Cones see color. But the picture they detect is upside down.
Why do we see the world right side up? "Thank your brain," says Maloney. It makes sense of the picture, flips it right side up, and tells you what you're seeing.
Splashes of Color
Cones allow us to view a multicolored world. Humans have three types of cones. When working together, they allow us to see a rainbow of colors, from violet to red.
Not all animals have this ability, says vision scientist Thomas Cronin of the University of Maryland. Most mammals, like dogs and cats, have only two types of cones. As a result, they see fewer colors than we do.
Other animals, like sharks, have only one type. Their world looks like a black-and-white movie.
For some animals, the world is even more colorful than it is for humans. Birds have a fourth type of cone that sees colors past the violet end of our rainbow--colors we can't see.
Mantis shrimp have eight or more different types of cones, the most of any animal. "Their rainbow is wider than we could ever imagine," says Cronin.
No animal can see in complete darkness, says Cronin. But some are far better than people at getting around in only the faintest light.
Who-who-who's afraid of the dark? Not owls. They hunt for food at night without a flashlight. "We would have trouble walking around in the same place," says Cronin. Fruit bats, cats, and other animals that are active at night also have great night vision.
How do they do it? Their eyes have many more rods than ours do, says Cronin. These light-sensing cells soak up light in dim conditions. As a result, an owl's eyes gather three to six times more light than a human's.
Lots of Lenses
Insect eyes, like those of flies and bees, are actually made of up to thousands of tiny eyes that work together. The eyes are packed close like a honeycomb. Each one looks in a slightly different direction. Compound eyes are made up of little lenses, each backed by its own light-sensitive cells.
All those extra eyes don't make an insect's sight any more focused, however. "A person can see the letters on a stop sign from far away," says Cronin, "but a fly will see only a bunch of blurry shapes."
The upside to compound eyes: They're great at detecting motion. No wonder it's so tough to nab a fly with a flyswatter!
Look Ma, No Eyes!
Imagine if you could sense light all over your body in the way that you sense touch. A sea star can.
Sea stars have light-sensitive cells all over their bodies. But without true eyes or a brain, sensing light is all a starfish can do. It can't look around to see rocks, fish, or a tasty clam lying in its path. Instead, it must rely on touch to find food and position itself over it.
We'll take our human eyes any day.
word to know
cell--the most basic part of a living thing
lens--a structure in the eye that focuses light
retina--a layer of eye tissue containing light-sensing cells
rods and cones--light-sensing cells named for their shape
compound eye--an eye containing up to thousands of parts, each creating a portion of an image
How Does the Human Eye See?
(1) Light bounces off an object and travels through the clear outer layer of the eye called the cornea.
(2) After the light passes through the cornea, it travels through the pupil. The pupil widens and shrinks to control the amount of light that enters the eye.
(3) The light that passes through the pupil then makes its way to the lens.
(4) The lens focuses the light onto a layer of tissue called the retina. There it forms an upside-down image. The brain interprets this image as upright. That's the image you see!
Lexile Level 750; Guided Reading Level N
Compare and contrast the eye structure of different animals.
Obtain six clear-plastic cups and arrange them in a circle on top of an overhead projector. Fill every other cup halfway with water. Add two drops of red, blue, or yellow food coloring to each cup with water in it.
1. Turn on the projector. Ask:
* What colors do you see? (red, yellow, blue)
* What term is used to describe these colors? (primary)
2. In each empty cup, mix the two primary colors on each side of it. Ask:
* What colors were just formed? (orange, green, purple)
* What term describes these new colors? (secondary)
* Which part of the human body senses color? (eyes)
* Which part of the eyes detects color? (cones in the retina)
* How does the structure of a shark's eyes differ from that of a human's? (Shark eyes contain one type of cone; they see in black and white. Human eyes contain three types of cones; they see in many colors.)
* Explain how certain animals, such as owls, have excellent night vision. (Owl eyes contain more rods These structures gather more light in dim conditions.)
For a Study Jams video and follow-up quiz, visit: http://studyjams.scholastic.com/studyjams/jams/ science/human-body/seeing.htm
READING AND LITERACY CONNECTION
Go to www.scholastic.com/superscience to download the skills sheet "What Do You Know?" Students complete a KWLS chart.
Common Core State Standard Reading Informational Text: 1
No-Sweat Bubble Test
Directions: Read each question below, and then use the article "Extraordinary Eyes" (pp. 4-7) to determine the best answer. Completely fill in the bubble next to the best answer.
1. Which part of the human eye controls how much light enters?
2. What is the function of the cones found in the retina?
(A) to gather light in dim conditions
(B) to allow animals to see in color
(C) to flip an image right side up
(D) to focus light onto the retina
3. Which part of the human eye is primarily responsible for focusing light onto the retina?
4. Which of the following animals possesses compound eyes?
(A) mantis shrimp
(B) sea star
5. Which sentence BEST describes why it is important for owls to have excellent night vision?
(A) Owls live and fly within forests.
(B) Predators hunt owls at night.
(C) Owls have no sense of hearing.
(D) Owls hunt for their prey at night.
6. Why are owls' eyes able to capture three to six times more light than a human's can?
(A) Owl eyes contain more rods.
(B) Owl eyes contain fewer rods.
(C) Owls have compound eyes.
(D) Human eyes contain more cones.
7. Which is the opposite of the word dim?
8. Which of the following animals does NOT see in color?
9. Which sentence BEST represents the main idea of this article?
(A) Some animals see the world with more color than humans do.
(B) Human eyes are superior to those of other animals.
(C) Animals active at night cannot survive without night vision.
(D) Different animals have different structures in the eye that affect how they see.
10. What was the author's purpose for writing this article?
(A) to compare the abilities of different types of eyes
(B) to describe how human eyesight works
(C) to review how the five senses are different for animals
(D) to show how animals' eyesight is more advanced than humans'
1. c 2. b 3. b 4. c 5. d 6. a 7. a 8. c 9. d 10. a
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|Title Annotation:||life science|
|Article Type:||Cover story|
|Date:||Nov 1, 2012|
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