CHAPTER 16 13 SECTION Sound and Light Reflection and Color KEY IDEAS As you read this section, keep these questions in mind: What happens to light when it hits an object? Why can you see an image in a? Why do we see colors? What Happens When Light Hits an Object? You may think of light bulbs, candles, and the sun as objects that send light to your eyes. However, all of the other objects that you see, including this book, also send light to your eyes. Otherwise, you would not see them. In order for you to see an object, light must travel from the object to your eyes. There are two main ways that light can get from an object to your eyes. Some objects, such as the sun or a light bulb, produce light. However, most objects, including this book, only reflect light from other sources. LIGHT AS A RAY It can be hard to imagine how light waves travel. To make it easier to understand and predict how light waves behave during reflection, refraction, and other processes, scientists use light rays. A light ray is an imaginary line that shows the direction in which light travels. The light ray s direction is the same as the direction the light wave travels. It is also the same as the path the photons of light take. When scientists draw light rays, they use single-headed arrows, like the ones shown below. READING TOOLBOX Compare After you read this section, make a chart comparing the images created when light is reflected from flat, concave, and convex s. 1. Define What is a light ray? Light ray Reflected light ray Light rays cannot be used to model all the properties of light. However, they can be useful for showing how light behaves in many situations. Refracted light ray Interactive Reader 359 Sound and Light
REFLECTION OF LIGHT Remember that waves can reflect, or bounce off, surfaces. All objects that you can see reflect at least some of the light that hits them. The way the light reflects affects what the surface looks like. Many surfaces, such as paper and skin, look dull. This is because light scatters off of them in many different directions, as shown in the figure below. This reflection of light into random directions is called diffuse reflection. During diffuse reflection, light rays reflect in many different directions. 2. Describe Would the surface in the figure look dull or shiny? Smooth, polished surfaces, such as s, look shiny. Instead of scattering, the light rays that hit smooth surfaces from a single direction all reflect in the same direction. You can see this in the figure below. Light rays that reflect off a smooth surface all travel in the same direction. 3. Describe What is the law of reflection? THE LAW OF REFLECTION Scientists describe reflection using angles. For example, look at the image at the top of the next page. The imaginary line that is perpendicular to the surface is called the normal. The angle between the incoming light ray and the normal is called the angle of incidence. The angle between the reflected light ray and the normal is called the angle of reflection. According to the law of reflection, the angle of incidence is equal to the angle of reflection. You can use this law to predict the direction of a light ray that reflects off a smooth surface. Interactive Reader 360 Sound and Light
Normal According to the law of reflection, the angle of incidence is equal to the angle of reflection. Incoming light Angle of incidence Reflected light Angle of reflection 4. Apply Concepts On the figure, draw an incoming light ray that has a smaller angle of incidence. Then, draw the reflected light ray for that incoming ray. Why Can You See Yourself in a Mirror? When you look into a flat, you see an image of yourself standing behind the. Of course, there is not really a copy of you behind the. The light rays that form the image seem to come from behind the, but they actually don t. Therefore, the image you see in the is a virtual image. The ray diagram in the figure below shows the paths of light rays striking a flat. When a light ray reflects off a flat, the light ray obeys the law of reflection. The angle of reflection equals the angle of incidence. When the reflected rays reach your eyes, your brain interprets the light as if it traveled in straight lines. So, you see an image of yourself behind the. In other words, flat s fool you into seeing objects that aren t actually there. 5. Identify What kind of image does a flat produce? The man sees a virtual image behind the. However, no light rays actually traveled from behind the to the man s eyes. This light ray reflects off the and travels to the man s eyes. His brain perceives the light as if it had traveled in a straight line. 6. Explain Why does the man see an image of himself in the? Visually, it appears that the reflected light ray traveled in a straight line from behind the. Interactive Reader 361 Sound and Light
7. Describe What do curved s do to reflected images? Think Critically In a small group, think of situations in which a curved would be more useful than a flat. Try to identify examples, other than the examples given here, of the uses of curved s in everyday life. 8. Compare How is a real image different from a virtual image? CURVED MIRRORS You are probably most familiar with flat s. However, people sometimes also use curved s. Curved s distort, or change, images they reflect. Like flat s, light reflects off of curved s according to the angle of reflection. However, the surface of a curved is not flat. Therefore, the normal to the points in different directions at different places on the. This results in the distorted images we see in curved s. There are two main kinds of curved s: convex s and concave s. Mirrors that bulge out are called convex s. Convex s produce virtual images that are smaller than the actual object. People use convex s to see large areas easily. For example, the on the passenger side of a car is a convex. This allows the driver to see a larger area of the road behind the car. Mirrors that curve inward are called concave s. People use concave s to focus reflected light onto a single point. A concave can form one of two kinds of images. It may form a virtual image behind the or a real image in front of the. Remember that light rays do not actually pass through a virtual image. A real image is an image that light rays pass through. The type of image that forms depends on the location of the object relative to the. Object Real image Mirror Light rays pass through a real image. Therefore, a real image would appear on a piece of paper placed in front of the. 9. Identify What determines whether the image produced by a concave is virtual or real? Mirror Front of Object Front of Back of Back of Virtual image Light rays do not pass through a virtual image. If you placed a piece of paper behind the, no image would form on the paper. Interactive Reader 362 Sound and Light
How Do We See Colors? White light from the sun contains all of the different wavelengths of visible light. What color an object appears to be depends on the wavelengths of light that come from it and enter your eyes. Your brain interprets different wavelengths of light as different colors. For example, when white light strikes a leaf, the leaf reflects only visible light with a wavelength of about 550 nm. The leaf absorbs light with other wavelengths. When the light reflected from the leaf enters your eyes, your brain interprets the light as green. Therefore, the leaf looks green. If you shine only red light on the leaf, the leaf looks black. This is because there is no green light for the leaf to reflect. 10. Identify What determines what color an object appears to be? MIXTURES OF COLORS Most of the colors that we see are not pure colors. They are mixtures of colors created by combining light or pigments. The additive primary colors of light are red, green, and blue. Mixing light of two of these colors can produce the secondary colors yellow, cyan, and magenta. Mixing light of the three additive primary colors makes white light. Filters and pigments, or dyes, absorb light. Therefore, mixing pigments or filters has a different effect on color than mixing light. The subtractive primary colors yellow, cyan, and magenta can be combined to create red, green, and blue. If pigments or filters of all three colors are combined in equal proportions, all visible light is absorbed. No light gets to your eyes, so you see black. Black is the absence of color. 11. Describe What color of light is produced by mixing red, green, and blue light? Magenta Yellow White Green Red Cyan Blue Red, green, and blue light can combine to produce yellow, magenta, cyan, or white light. Cyan, magenta, and yellow filters or pigments can combine to produce red, green, blue, or black. 12. Explain Why does a mixture of yellow, cyan, and magenta pigments look black? Interactive Reader 363 Sound and Light
Section 3 Review Section Vocabulary light ray a line in space that matches the direction of the flow of radiant energy real image an image that is formed by the intersection of light rays; a real image can be projected on a screen virtual image an image from which light rays appear to diverge, even though they are not actually focused there; a virtual image cannot be projected on a screen 1. Explain Why do scientists use light rays to represent light waves? 2. Identify Give three examples of objects that produce diffuse reflection. Explain how you know these objects produce diffuse reflection. 3. Identify Label the normal, angle of incidence, and angle of reflection in the diagram below. 4. Apply Concepts The angle between an incoming light ray and the normal is 25. What is the angle of reflection? 5. Infer What color would a blue object look if you shined red light on it? Explain your answer. Interactive Reader 364 Sound and Light