FOCUS Book How does the amount of light around you affect your eyes? Make a hypothesis and write it down. Study your eyes in a mirror, looking at the iris and pupil. Make a sketch of one eye, labeling the different parts. Dim the lights. Look at your eyes again in the mirror. Make a sketch of what you see. Describe any differences. INCREDIBLE EYES What else can you observe about your eyes? Think of another question you could ask and design a test that will answer it. NOTE: Do not shine bright lights into your eyes, and never ever look into the Sun! Try your test. Record your observations and analyze your results. Did they match your hypothesis? Write a summary of your findings. Be sure to include evidence from what you observed. Beyond the Book How do animals other than those in this book use their eyes? Which ones have special features or abilities? Learn more about animal eyes online or in the library.
INCREDIBLE EYES FOCUS Question How do eyes help animals survive? Structure and Function Photo Credits: Front cover: Rolf Nussbaumer Photography/Alamy; page 2 (top): Sinibomb Images/Alamy; page 2 (bottom): Sunabesyou/Dreamstime.com; page 3 (main): Hill Street Studios/Blend Images/Corbis; page 3 (rose inset): Igor Terekhov/123RF; page 4 (top): Blend Images/Alamy; page 4 (bottom): Theo Gottwald/Dreamstime. com; page 5 (top): Bruce Macqueen/Dreamstime.com; page 5 (bottom): Ribe/Dreamstime.com; page 6 (top): Stephen Noakes/Dreamstime.com; page 6 (bottom): Ray Kasprzak/123RF; pages 7, 9 (bottom): FLPA/Alamy; page 8 (top): blickwinkel/alamy; page 8 (bottom): Phil Degginger/Alamy; page 9 (top): Martin Mcmillan/Dreamstime.com Why Eyes? If you look at your eyes pupil in the mirror, you will see the pupil and the iris. The pupil is the dark hole in the center. The iris is the colored part. These two parts work together to let light into the eye. Have you tried to take a photo in a dark room? The picture will look better if you use a flash. Light from the flash reflects off the items in the room. Reflected light enters the camera and the camera records the image. Animal eyes do something similar. They take in reflected light and the brain makes sense of the image. iris Incredible Eyes Learning A Z All rights reserved. www.sciencea-z.com 2
How the Eye Works When reflected light enters the eye, it passes through the cornea, a thin, clear covering. Next, the light goes into the pupil. Eye muscles on the iris control the pupil by making it larger or smaller. This lets in more or less light. The lens is the curved structure in your eye that bends light to focus an image onto the back of the eye. In the back of the eye is the retina. The image that forms there is actually upside down! The brain fixes this and flips the image to make it right-side-up. retina HOW A HUMAN EYE SEES lens cornea Roses Are Red What makes a green leaf look green? The answer begins with understanding sunlight. You can make a rainbow with a prism, which separates the Sun s white light into the many colors we see. When sunlight hits a leaf, some light is When you see a green leaf, it is because only green light is reflected off the leaf into your eye. absorbed and some is reflected. A green leaf absorbs all the colors of light except green. The reflected green light is the color you see. a prism iris pupil Your eye has small cells in the retina called cones that respond to different colors of light. When you see green, the cones that react to the color green send signals to your brain. Your brain processes that information and tells you that you are seeing green. This diagram shows how the human eye works. Other animal eyes work in a similar way. Some animals, such as snakes, vampire bats, and bedbugs, can see heat. One form of heat is infrared light, which is outside the spectrum of light that humans can see. Inside Living Things Incredible Eyes 3 4
Seeing in the Dark Unlike people, cats are most active at night. How do they see in the dark? Cats have a shiny, reflective surface inside their eyes. This surface reflects light onto the retina to help cats see in the dark. At night, their pupils open up to let in more light. Other nocturnal animals, like deer, have similar eyes. Deep-sea animals like the giant squid and the octopus live in darkness. They have some of the largest eyes of all animals. Large eyes can let more light in to help these animals see in the dark. cat eyes Eat or Be Eaten One of the biggest reasons animals need vision is to be able to hunt for food and to avoid being hunted. Animals that hunt are called predators, and the animals they eat are called prey. Most predators, including owls, have eyes on the front of their heads. This lets them focus both eyes forward, giving them stereoscopic (STEH-ree-oh-SKOP-ick) vision. Their two eyes create a single image. This ability helps animals tell how far away an object is. Try covering one eye. It s hard to figure out how near or far something is without stereoscopic vision. predator eyes The owl s prey, a squirrel, has eyes on the sides of its head. This allows the squirrel to spot predators on all sides without octopus eye moving around and risking getting noticed. Inside Living Things Incredible Eyes 5 6 the eyes of a prey animal
Swivels and Stalks A crab s eyes are on top of movable stalks. The stalks let the crab see in all directions. Each eye swivels around to track nearby movement. A chameleon s eyes can also turn in different directions. Each eye rotates on its own! Crabs and chameleons can see all the way around their bodies. Yet both can also focus their eyes in the same direction, giving them stereoscopic vision. They can scan for meals and avoid being a meal at the same time. Though crab and chameleon eyes look different, they help each animal in similar ways. eyes eyestalks Compound Eyes Arthropods, such as crabs, insects, and spiders, have compound eyes. Compound eyes are made up of many small parts. The fly on this page has two giant eyes. Each eye is actually hundreds of tiny eyes. Compound eyes are made of groups of cells called ommatidia. The basic unit of the compound eye is a group of cells that forms an image. Some insects have fewer than a dozen of these groups. Dragonflies have many thousands! When a dragonfly sees the world, its brain has to make sense of thousands of different images. The insect brain might be small, but it is not simple. eyestalks on a crab A jumping spider has multiple pairs of compound eyes. It spots prey with two sidewayspointing eyes. Then it homes in with an extra pair of large, front-facing eyes. Inside Living Things Incredible Eyes 7 8
Eye Protection Eyes are important for survival, so it pays to keep them safe. We wear sunglasses to keep the sunlight and dust out of our eyes. Some animals have eyelashes and eyelids to protect their eyes. Camels have double rows of eyelashes to keep out sand and dust. They also have bushy eyebrows to shield their eyes from sunlight. Fishes, frogs, snakes, and many other animals have a special membrane. This is a clear covering that prevents scratches and other harm to their eyes. A gecko can clean its own eyes by licking them. bushy eyebrows inner eyelashes outer eyelashes the eye of a camel Write your answers on separate paper. Use details from the text as evidence. 1 What are the basic parts of a human eye, and how do they work like a camera? 2 After leaving a dark movie theater on a Saturday afternoon, you step out into the bright sunshine. What happens in your eyes? Why? 3 Examine the prism on page 4. Explain why a red rose appears red. 4 Compare and contrast the eyes of a cat with the eyes of an octopus. How do the structures of each animal allow it to see in the dark? 5 How do a fly s compound eyes work to make this insect extremely difficult for other animals to catch? FOCUS Question How do eyes help animals survive? Study the photos of a predator (owl) and prey (squirrel) on page 6, as well as an animal that can be both predator and prey, such as the crab on page 7. Explain how the specific features of their eyes help keep each animal alive. Inside Living Things Incredible Eyes 9 10