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Photo Credits: Cover: Digital Vision/PictureQuest; p. 2: Corbis Sygma; pp. 4–5: Royalty-Free/Corbis; p. 6: NASA/STSci Cornell/Corbis, Corbis; p. 7: NOAO; pp. 8–9: Getty Images; p. 10: NASA/Photo Researchers, Inc.; p. 11: Time Life Pictures/Getty Images; p. 12: David Ducros/Photo Researchers, Inc. Copyright © by Harcourt, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be addressed to School Permissions and Copyrights, Harcourt, Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887-6777. Fax: 407-345-2418. HARCOURT and the Harcourt Logo are trademarks of Harcourt, Inc., registered in the United States of America and/or other jurisdictions. Printed in the United States of America ISBN-13: 978-0-15-352396-0 ISBN-10: 0-15-352396-4 1 2 3 4 5 6 7 8 9 10 175 15 14 13 12 11 10 09 08 07 06 If you have received these materials as examination copies free of charge, Harcourt School Publishers retains title to the materials and they may not be resold. Resale of examination copies is strictly prohibited and is illegal. Possession of this publication in print format does not entitle users to convert this publication, or any portion of it, into electronic format.
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Time or Distance? “Steer the ship to the moons of Ptolemy 5, Lieutenant.” “But those are 3 light years away, Captain!” “Exactly. Put on your seatbelt. It’s going to be a long trip.” You’ve probably heard this kind of conversation on TV or in a movie that takes place in outer space. What is a light year: time or distance? It’s sort of both. A light year measures distance, but the measurement it uses is the distance light would travel in a year in a complete vacuum--empty space. Because light is so fast, it would go very far in that time. Scientists use the term because it is easier to say “1 light year” than 9,460,730,472,580,800 meters, which is the same distance. That’s almost 9.5 quadrillion meters. Earth is much closer to our sun than a light year. As a matter of fact, they’re only about 8.3 light minutes apart. That means light leaving the sun takes about 8.3 minutes to reach Earth. The moon is even closer: about 1.3 light seconds away. 2
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Beam Me Up! Besides being fast, light is known for how straight it travels. It’s not just as “straight as an arrow;” it’s much, much straighter. One form of light, lasers, are often used to make sure things are lined up correctly. Light travels in such a direct line that scientists on Earth can a shoot a laser beam up to reflectors left on the moon by astronauts...and the beam bounces right back to where it was sent. That’s a journey of about 750,000 kilometers. Talk about straight! This laser at the University of California’s Lick Observatory helps astronomers focus their telescopes. Astronauts left this special reflector on the moon in 1969. It still works...and it’s smaller than your desk! 3
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A Clown Car of Color Have you ever seen a routine where clowns keep coming out of a tiny little car? Light from the sun is a lot like that car: it packs a lot in. What appears to be white light to us is actually bursting with colors. The colors in light become visible when the light hits a prism. The prism refracts--or bends--the light and produces a spectrum of colors. From the top to the bottom, the spectrum is red, orange, yellow, green, blue and violet. The spectrum actually has every color you can imagine... and some you can’t. We can’t see ultraviolet, for example, but we can see (and feel!) what it does to us: sunburn. Just like water in the ocean, light travels as waves. The waves all travel at the same speed, but some are very close together, and some farther apart. The wavelength of light determines its color. Red has the longest wavelength of the colors we see, and violet has the shortest. That’s why they appear on opposite ends of the spectrum. The shorter the wavelength, the more the light gets refracted by the prism. Visible Light Region of the Electromagnetic Spectrum PM PM PM PM )NFRARED 5LTRA6IOLET 4
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Light often gets refracted when it moves from one medium, such as air, to another, such as a glass prism. Even drops of water can refract light into colors. When this happens in the sky, we get a rainbow. Not all refraction splits light into its colors, though. Some just seems to bend the light. If you put a straw half into a glass of water, for example, the straw will appear bent or broken because the water refracts the light and makes the straw appear in a different place. The straw hasn’t changed, though: the light has.> <The glass of water refracts light to make this straw look broken. 5
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A lens will also refract light, but usually without breaking it into separate colors. We use lenses in glasses, contact lenses, telescopes, microscopes and many other everyday things. A magnifying glass refracts light coming through it to a small focus point. It is called a converging lens because the light beams coming through it converge--or meet--at a point. Many other objects--like those special reflectors left on the moon--reflect light. A mirror is so reflective and smooth that you can see yourself in it. When an object is rough or bumpy, light scatters off its surface in all directions. That’s why you can see yourself in a mirror, but not in a crumpled piece of aluminum foil. %&&%#4/&#/.6%2').'#/.6%8 ,%.3 "EAMOFLIGHT &0 &0 &0&OCUS0OINT 6
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Blue Noon When scientists talk about light’s speed and other properties, they often talk about how light behaves in a vacuum. A vacuum is space that is so empty, there’s not even air there. But we don’t live in a vacuum; that’s why we have blue skies and red sunsets! As light moves through Earth’s atmosphere, particles and gases cause some of the light to scatter, just like when they hit a rough surface. Light with shorter wavelengths--the blue end of the spectrum- -gets scattered much more than light with longer wavelengths. Blue light scatters so much that it appears to come from all directions. That’s why the sky seems blue--at least between sunrise and sunset. When the sun is rising or setting though, its light passes through much more of Earth’s atmosphere than in the middle of the day. As a result, the blue end of the spectrum gets scattered almost completely out of the sunlight. That leaves the yellows, oranges and reds to shine through brightly...pretty as a postcard. Wish you were here! San Diego, CA 7
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A World of Color If sunlight appears “white” or colorless, why do you think our world is full of colors? The answer may surprise you. Think an apple is red? Think again. Prisms and pure water are transparent, meaning most light passes through them. Other objects, such as a sheet of paper or a frosted light bulb, are translucent: they allow only some light to pass through. But apples and most of the things we see as colored are opaque. They don’t allow light to pass through. What happens instead? Light either bounces off them--reflection--or it gets absorbed into them like a sponge sucks up water. And when light hits them from one side, a shadow appears on the other side as a dark area. Our world doesn’t appear to be just shades of light and dark, though. It’s colorful! That’s because most objects absorb or reflect certain wavelengths of light. So that must mean an apple absorbs red, right? Actually, apples reflect red and absorb every other color--that’s why they appear red to us. White light hits the apple, the apple absorbs every color but red, and red light bounces off the apple and into our eyes. Oranges absorb 8
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red and every other color except orange, which they reflect into our eyes. Bright objects reflect more light, and dark objects less light. Not all light is white, though. Think of the trouble we would have if traffic lights didn’t have the red, green, and yellow light to tell drivers what to do! Lasers often shine in a very specific shade of color that depends on the material used to produce them. A ruby laser, for example, has a wavelength of about 694 nanometers— that’s “red” to you and me. 9
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Focus on the Eye The eye is an amazing tool for seeing light. Light enters the eye through the pupil, which is the dark area in the center of the eye. The iris, which is the colored ring around the pupil, is actually a muscle. It opens the pupil up wide to let in more light when it’s dim, and closes it to a tiny size when it’s bright to protect the inner eye from too much light. Like sunglasses you were born with! After light enters through the pupil, it passes through the lens. It’s just like a glass converging lens, but much more sensitive. The lens can actually change its own shape to focus more accurately on the back of the eye, called the retina. The retina has two kinds of light- sensing cells, called rods and cones. These photoreceptor cells sense all the light that makes up what we “see.” retina lens pupil iris 10
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There are about 20 times as many rods as cones. The rods can’t really sense color, but they are more sensitive to light and help us to see at night. Cones sense color and come in three kinds. One kind is very sensitive to reddish light, another to bluish light, and the third to greenish light. The rods and three types of cones communicate what they sense to the brain through a nerve at the back of the eye. The brain pieces all the information together and that’s what we see. The Earth has so many artificial lights that it can be hard to see faint stars and other dim objects. This picture shows Earth at night. Can you see where there might be problems with ‘light pollution?’ 11
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The eyesight of individuals varies greatly. What is 20/20 vision? It’s a measure of visual acuity. If you have 20/20 vision, it means you can see small details from 20 feet away as well as someone with standard vision can from 20 feet. So, 20/40 vision is not as sharp: it means you can see the same details at 20 feet as someone with standard vision can see from 40 feet away. Someone lucky enough to have 20/15 vision can see as well at 20 feet away as a person with standard vision can see at 15 feet. Many animals can see better than humans in one way or another. Bees and butterflies can see ultraviolet. Some snakes can see infrared--which is beyond red on the other end of the spectrum. Birds have at least four different cones in their retinas--so they can distinguish colors better within our range--and many can also see in the ultraviolet range. The visual acuity of hawks may be as good as 20/2. That means they can see details as well at 20 feet as most humans can see at 2 feet! Humans don’t have the eyes of these animals, but we have something they don’t: the ability to make tools such as telescopes, ultraviolet cameras, infrared (or “night vision”) goggles and other devices that greatly expand what we can see in the world. And that’s out of sight! 12
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Think and Write 1. How long would it take for a laser to leave Earth, hit reflectors on the moon, and return to Earth? How do you know? 2. Explain the difference between reflection and absorption. 3. Why do you think scientists might develop ultraviolet cameras and infrared goggles? 4. Persuasive Writing Write one paragraph arguing that light pollution is just as bad as air and water pollution...or arguing that light pollution is not as bad as other pollution. Hands-On Activity Quick Flick Use a meter stick to measure the length of your room. If it takes a flicker of light one three- hundred-millionth of a second to travel one meter, about how long would it take light to cross your room? Put the length of the room in meters over 300, reduce the fraction if you can, and then add six zeroes to the bottom number. That will show how many seconds it will take. School-Home Connection Eyes on the Prize Survey your family members to find out if they wear glasses or contact lenses. With your class, make a chart showing how many people wear glasses or contacts.
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