# 3 Interactions of Light Waves

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1 CHAPTER 22 3 Interactions of Light Waves SECTION The Nature of Light BEFORE YOU READ After you read this section, you should be able to answer these questions: How does reflection affect the way we see objects? What happens to light when it passes from one material into another? How do light waves interact with each other? National Science Education Standards PS 3c What Is Reflection? Light bounces off some objects and passes through others. Reflection happens when light bounces off an object. Light reflects off objects all around you. You see the objects because the reflected light travels to your eyes. You can use the law of reflection to predict where a light wave will travel after it reflects. The law of reflection states that the angle of incidence of light is equal to the angle of reflection. The angle of incidence is the angle at which a light beam hits an object. The angle of reflection is the angle at which the light bounces off the object. The law of reflection states that these two angles are equal. The Law of Reflection The beam of light traveling toward the mirror is called the incident beam. A line perpendicular to the mirror s surface is called the normal. The beam of light refl ected off the mirror is called the refl ected beam. STUDY TIP Organize Concepts As you read, make a Concept Map in your notebook. Try to include the vocabulary terms and the terms in italics. 1. Apply Ideas If a light beam hits a mirror at a 30 angle, what is the angle of incidence? 2. Apply Ideas What is the angle of refl ection? The angle between the incident beam and the normal is called the angle of incidence. The angle between the refl ected beam and the normal is called the angle of reflection. Interactive Textbook 415 The Nature of Light

2 REGULAR REFLECTION AND DIFFUSE REFLECTION Light reflects off most objects, such as mirrors and walls. Why can you see your reflection in a mirror, but not in a wall? The answer has to do with the surface the light reflects from. Some objects, such as mirrors, reflect all the light beams at the same angle. This is called regular reflection. Regular reflection allows you to see an image in a mirror or shiny piece of metal. Some objects, such as walls, reflect the light beams at many different angles. This is called diffuse reflection. Diffuse reflection prevents you from seeing images reflected off walls, furniture, or other rough surfaces. 3. Identify Circle the image that shows how light refl ects from a mirror. 4. Compare and Contrast What is the difference between a luminous object and an illuminated object? Regular reflection occurs when light beams are refl ected at the same angle. When your eye detects the refl ected beams, you can see a refl ection on the surface. Diffuse reflection occurs when light beams refl ect at many different angles. You can t see a refl ection because not all of the refl ected light is directed toward your eyes. SEEING OBJECTS You can see some objects because they produce light. You can see other objects because they reflect light. Luminous objects are light sources. Flames, light bulbs, and the sun are examples of luminous objects. Illuminated objects are visible because they reflect light. The reflected light travels from the objects to your eyes. The moon is an illuminated object. You see the moon because it reflects light from the sun to your eyes. You can see the body of a fi refl y because it is illuminated. You can see the tail of a fi refl y because it is luminous. Interactive Textbook 416 The Nature of Light

3 How Is Light Absorbed and Scattered? When you shine a flashlight, you can observe the light beam spread out and become dim farther from the flashlight. The light beam becomes dim because of absorption. It spreads out because of scattering. LIGHT ABSORPTION Absorption is the transfer of light energy into an object or a particle of matter. Air is full of particles, such as water droplets or dust. When light shines through the air, these particles absorb some of the light energy. The farther light travels from its source, the more its energy is absorbed by particles. Therefore, light is dimmer farther from its source. 5. Explain What makes a beam of light become dimmer as it travels away from its source? When light is absorbed by a particle, the light beam loses energy and becomes dim. LIGHT SCATTERING Scattering is the interaction of light with an object that changes the light s direction or energy. When light collides with a particle in the air, the light scatters in all directions. The scattering of visible light makes the sky look blue. Light with shorter wavelengths is scattered more than light with longer wavelengths. Blue light is scattered more by the atmosphere than red light. The red light passes more easily through the atmosphere. Therefore, the sky looks blue. 6. Explain Why is the sky blue? When light collides with a particle, it scatters in all directions. Scattering allows you to see objects outside a beam of light. Interactive Textbook 417 The Nature of Light

4 What Is Refraction? Refraction happens when a wave bends as it passes from one material into another. Light travels at different speeds through different materials. Therefore, passing from one material to another can change its direction. Light travels faster in a vacuum than in matter. Light travels faster in air than in glass or water. Light bends away from the boundary between the materials here because light travels more slowly in glass than in air. 7. Apply Concepts Which way will light bend when it passes from air to water? Explain your answer. Light in Light bends toward the boundary between the materials here because light travels faster in air than in glass. Critical Thinking 8. Infer What is an optical illusion? OPTICAL ILLUSIONS Light from a light source or reflected from an object travels in a straight line through the air to you. Your brain expects light to always travel in a straight line. However, when you look at an object under water, the light refracts, or bends. Because it doesn t travel in a straight line, you may see an optical illusion, such as the one shown below. Air Water Air Water Because of refraction, the cat and the fi sh see optical illusions. To the cat, the fi sh appears closer than it really is. To the fi sh, the cat appears farther away than it actually is. Interactive Textbook 418 The Nature of Light

5 SEPARATING COLORS White light contains all of the colors of visible light. Each color has a different wavelength. Different wavelengths travel at the same speed in a vacuum, but they travel at different speeds in glass. Therefore, some colors refract more than others when they travel through the glass. Colors with the shortest wavelengths refract the most. Colors with the longest wavelengths refract the least. Light passing through a prism is refracted twice once when it enters and once when it exits. Red light has a longer wavelength than other colors. Red light is refracted the least. Violet light has a shorter wavelength than other colors. Violet light is refracted the most 9. Identify What happens to light when it hits a prism? A prism is a piece of glass that uses refraction to separate white light into the colors of visible light. What Is Diffraction? Diffraction happens when waves bend around a barrier or through an opening. The amount a wave bends depends on two things: its wavelength and the size of the barrier or opening. Waves diffract the most when the barrier or opening is the same size as the wavelength or smaller. The wavelength of visible light is very small. It is about 100 times thinner than a human hair. Therefore, a light wave won t bend much unless it passes through a very narrow opening or around a sharp edge. Critical Thinking 10. Compare and Contrast How are diffraction and refraction of light alike? How are they different? DIFFRACTION YOU CAN SEE You will not often see light waves diffracting around objects. For example, you can t see the objects that are around the corner of a building. However, you can see some effects of light diffraction. For example, diffraction makes the edges of a shadow look blurry. Interactive Textbook 419 The Nature of Light

6 11. Describe Compared to the wavelength of the light, how large is the disk? This image shows light diffracting. A single wavelength of light is passing around the sharp edge of a tiny disk. 12. Identify What are the two kinds of interference? What Is Wave Interference? Interference happens when two or more waves combine to form a single wave. Interference occurs when two waves overlap. When waves overlap, they can combine by constructive interference or destructive interference. Constructive interference produces a wave with a larger amplitude than the original waves. Waves that combine by constructive interference create one wave with a stronger intensity. For example, light waves that combine by constructive interference produce one brighter light wave. Destructive interference produces a wave with a smaller amplitude than the original waves. Waves that combine by destructive interference create one wave with a weaker intensity. For example, light waves that combine by destructive interference produce one dimmer light wave. Constructive and Destructive Interference 1 Light of a single wavelength 2 The light waves diffract as they passes through two slits. pass through the tiny slits. 13. Identify What kind of interference produces the dark bands? 3 The diffracted light waves 4 The interference can be seen on a combine by both constructive and destructive interference. screen as bright and dark bands. The bright bands are the result of constructive interference. The dark bands are the result of destructive interference. Interactive Textbook 420 The Nature of Light

7 LIGHT INTERFERENCE YOU CAN SEE The experiment shown in the last figure used only one wavelength of light. However, the light you see around you every day is white light. White light contains many different wavelengths. The many waves don t usually combine in total destructive or constructive interference. This is why you do not usually see the effects of interference. Light Wave Interaction 14. Explain Why do we not usually see the effects of constructive and destructive light interference? 15. Describe Complete the Flow Chart with the information from this section. Interactive Textbook 421 The Nature of Light

8 Section 3 Review NSES PS 3c SECTION VOCABULARY absorption in optics, the transfer of light energy to particles of matter diffraction a change in the direction of a wave when the wave fi nds an obstacle or an edge, such as an opening interference the combination of two or more waves that results in a single wave reflection the bouncing back of a ray of light, sound, or heat when the ray hits a surface that it does not go through refraction the bending of a wavefront as the wavefront passes between two substances in which the speed of the wave differs scattering an interaction of light with matter that causes light to change its energy, direction of motion, or both 1. Explain Why is a beam of light wider and dimmer when it hits a wall than when it leaves the flashlight? 2. Describe Use the vocabulary words to name the type of wave interaction that causes each condition. Condition Wave Interaction A log is sticking up from the water. It looks as if its top and bottom are not attached. The edge of a shadow is blurry. You can see your image in the mirror. A beam of light becomes dimmer farther away from its source. 3. Apply Ideas In a dark room, you can see the image on the screen of a television. However, you cannot see the table the television is sitting on. Why can you see the image but not the table? 4. Explain Why can you see your reflection in the bowl of a spoon but not in a piece of cloth? Interactive Textbook 422 The Nature of Light

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