Chapter 2 • Lesson 6
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5
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The Behavior of Light • medium • reflection • mirror • refraction • prism
Getting the Idea In Lesson 5, you learned that light is energy you can see. You can see objects around you because light bounces off them into your eyes. But different objects and materials cause light to act in different ways.
How Light Travels Light travels in waves. The waves travel away from a source, such as the sun or a lightbulb, in all directions. Light travels in a straight line until it hits something. Light travels through empty space as well as through some materials. A material through which light travels, such as air or glass, is called a medium. You can draw light as an arrow or line to show its path. The diagram below shows three things that can happen when light strikes an object. You will learn about these processes as you read this lesson.
Reflection
Refraction
Absorption
Reflection What happens when light strikes an object depends on what material the object is made of. Some materials let light pass through them. Others reflect, or throw back, light. They cause light to bounce off them. The bouncing of light off a surface is called reflection. Everything we see reflects some light. The light bounces off the surface of the object and into our eyes, so we see the object.
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You probably look in the mirror each day to brush your hair or teeth. A mirror is an object with a very smooth surface that forms images by reflecting light. Most mirrors are sheets of glass with a silver coating on the back. When you look in a mirror, your body reflects light into the mirror and the light bounces back into your eyes. You see yourself because the angle at which light strikes the mirror is the same size as the angle at which the light is reflected. Reflected light
Mirror
Refraction When light passes from one material into another, the light changes direction. This bending of light is called refraction. Refraction happens because light slows down or speeds up when it passes into a different material. Clear plastic, glass, air, and water all refract light. If you put a pencil in a glass of water, the pencil looks bent or broken. That is because light is refracted as it passes from air into water. Refraction Look at the diagram of a prism below. A prism is a clear, or seethrough, object that separates white light into all the colors of the rainbow. Most prisms, such as the prism in the diagram, have three sides
White light
Red ' Orange Yellow Green >• Spectrum Blue Indigo Violet j
A prism separates white light into its individual colors.
A prism refracts light. When white light enters the prism, the prism refracts each color differently. The prism bends each color by a different amount. In the diagram above, you can see that red light is bent less than orange light. Orange light is bent less than yellow light. All the colors that make up white light are called the spectrum. You may know someone who wears eyeglasses. The clear glass or plastic pieces in eyeglasses are lenses. A lens is a clear object that has at least one curved surface.
40 • Chapter 2: Energy, Forces, and Motion
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Lesson 6: The Behavior of Light
A lens refracts the light that passes through it. Lenses bend light in ways that help people see better. A hand lens refracts light to make objects appear larger. Lenses are also used in microscopes and telescopes. The Senses in microscopes help make things look hundreds or even thousands of times larger than they really are. These tools help scientists make observations of tiny living things.
Absorption All materials absorb, or take in, some light. When a material takes in light energy, the material changes the light energy into heat. This process is called absorption. Dull materials absorb more light than they reflect. Shiny materials reflect more light than they absorb.
F o c u s on Inquiry I Scientists use their senses to make observations. They also build and use tools that help them use their senses. People have used lenses for hundreds of years. Lenses have helped scientists make observations and discoveries. Antony van Leeuwenhoek and Galileo Galilei were two scientific investigators who used lenses. Do research in a library or on the Internet. Find out how these two scientists used lenses. Find out what discoveries and contributions they made to the field of science. On separate paper, write a summary of your research. You may want to include diagrams and artwork. Look for sources of information that can be trusted. You might start with an encyclopedia. Or you might try a nonfiction book written by a scientist. Choose the newest sources you can find. Older sources might contain out-of-date information. Remember that not all information on the Internet is true. Look for Web sites that are run by people who check their facts. These include college, government, and museum Web sites. Ask a teacher, librarian, or family member to help you find Web sites you can trust. You can also use a search engine, such as Google or Yahoo, to locate sites related to your topic. Then study the sites carefully before using facts from them. Make sure that a trustworthy group runs the site. Look to see if the information has been updated recently. When you write about your research, be sure to put the information into your own words. You should never copy directly from a source. Using your own words will also help you understand what you have read. 41
L e s s o n Review 1.
Which object works by reflecting light? A. mirror B. prism C. lightbulb D . hand lens
2.
What happens when a material refracts light? A. Light rays bounce off the surface of the material. B. Light rays are bent as they pass through the material., C. Light rays stop moving. D . Light energy is changed into heat.
3.
Which of these would NOT be a trusted source of information about light? A. a fiction book about a boy who uses a prism B. your science textbook C. an encyclopedia entry about light D . your science teacher
4.
Which scienrist is known for using lenses to help make scientific discoveries? A.
Newton
B. Einstein C. Archimedes
D . Galileo
42 • Chapter 2: Energy, Forces, and Motion
