CHAPTER 16 14 SECTION Sound and Light Refraction, Lenses, and Prisms KEY IDEAS As you read this section, keep these questions in mind: What happens to light when it passes from one medium to another? How do lenses work? How can a prism separate white light into colors? What Happens When Light Passes from One Medium to Another? Light, like all waves, travels in a straight line in a single medium. However, remember that waves refract, or bend, when they move from one medium to another. Like all waves, light bends when it moves from one medium to another. It does this because the speed of light is different in each medium. The amount a light wave bends depends on the relative speeds of light in each medium. If light slows down when it enters a new medium, the light ray bends toward the normal. If it speeds up when it enters a new medium, it bends away from the normal. For example, the speed of light is higher in air than in glass. When light moves from air to glass, the ray bends toward the normal. When light moves from glass to air, the ray bends away from the normal, as shown below. READING TOOLBOX Compare and Contrast As you read this section, make a chart to compare and contrast how light interacts with mirrors, lenses, and prisms. READING CHECK 1. Explain Why does light refract when it moves into a different medium? Glass When light moves from air into glass, the light slows down. The light ray bends toward the normal. Glass When light moves from glass into air, the light speeds up. The light ray bends away from the normal. 2. Describe What happens to the speed of light when light moves from air into glass? REFRACTION AND VIRTUAL IMAGES Remember that a virtual image is an image that light rays do not pass through. Refraction of light, like reflection off of a flat mirror, can produce virtual images. The figure at the top of the next page shows an example of this. Interactive Reader 365 Sound and Light
Virtual image Water Water 3. Apply Concepts In which substance is the speed of light greater, air or water? Light bends away from the normal when it moves from the water into the air. Therefore, the cat sees a virtual image of the fish that is higher in the water than the fish actually is. Light bends toward the normal when it moves from the air into the water. Therefore, the fish sees a virtual image of the cat that is higher above the water than the cat actually is. 4. Explain If you look into a body of water, such as a lake, from a dock or boat, the water may look shallower than it actually is. What do you think is the reason for this? The images that the cat and the fish see are virtual images. Light bends away from the normal when it passes from the water to the air. The cat s brain interprets the light as if it traveled in a straight line. Thus, the cat sees a virtual image of the fish. Similarly, light bends toward the normal as it passes from the air into the water. Thus, the fish sees a virtual image of the cat. How Do Lenses Work? You may not realize it, but you use the refraction of light every day. Human eyes, as well as cameras, contact lenses, eyeglasses, and microscopes, contain parts that bend light. When light travels at an angle through a thin, flat medium, such as a pane of glass, it is refracted twice. First, it is refracted when it enters the medium. Second, it is refracted when it leaves the medium. The light ray s position as it exits the medium is shifted, but it is still parallel to the original light ray. If the medium has a curved surface, the exiting rays will not be parallel to the original ray. The light will be traveling in a different direction after it leaves the medium. Therefore, a curved piece of a transparent medium, such as glass, can change the direction in which light travels. Such curved mediums are called lenses. Interactive Reader 366 Sound and Light
TYPES OF LENSES There are two main kinds of lenses: converging lenses and diverging lenses. A converging lens, which is thicker in the middle than at the edges, bends light inward. This type of lens can create either a virtual image or a real image. The type of image depends on the distance from the lens to the object. A converging lens focuses light inward. The light rays focus to a point called the focal point of the lens. A magnifying glass is an example of a converging lens. If you hold the lens near the object, you see a larger image of the object through the lens. If you hold the lens farther from the object, you see a smaller image of the object. A converging lens, such as a magnifying glass, can produce different magnifications. Magnification is any difference between the size of an image and the size of the object. A magnification of an object is larger than the object. A diverging lens is thinner in the middle than at the edges. It bends light outward and can create only a virtual image. Looking CLoser 5. Identify On the figure, circle the focal point of the lens. Talk About It Identify Relationships Look up the words converge and diverge in a dictionary. In a small group, talk about why different lenses are called convergent lenses or divergent lenses. A diverging lens bends light outward. It can produce only virtual images. Looking CLoser 6. Describe What type of image can a diverging lens produce? MICROSCOPES A compound light microscope uses multiple lenses to provide greater magnification than a single magnifying glass can. The objective lens is close to an object and forms a large, real image of the object. The ocular lens in the eyepiece acts like a magnifying glass. It magnifies the real image from the objective lens. This creates the large, virtual image you see when you look through the microscope. Interactive Reader 367 Sound and Light
How Do Human Eyes Work? Think about a camera that uses film. Light enters the camera through a large lens. The lens focuses the light into an image on the film at the back of the camera. Parts of the human eye work in very similar ways to these parts of a camera. The figure below shows how the different parts of the eye affect light. 7. Explain What can change how much light refracts when it passes through the lens? How the Eye Works 3 The light is refracted again by the lens. Muscles around the lens can change its shape to affect how much the light is refracted. 2 After it passes through the cornea, the light passes through the pupil. The pupil is the dark hole in the center of the colored part of the eye. 1 The cornea is a transparent membrane that covers the eye. The cornea refracts light. 4 The refracted light is focused onto the back surface of the inside of the eye, which is called the retina. 5 Special structures on the retina called rods and cones detect the light. Cones detect colors, but respond only to bright light. Rods detect dim light, but cannot resolve details well. 6 The signals from the rods and cones are carried to the brain by the optic nerve. The brain interprets the signals to determine what you are looking at. How Do Prisms Separate White Light Into Colors? Remember that white light contains many different wavelengths, or colors, of visible light. A prism can separate white light into its component colors. A prism is a transparent solid that is generally made of glass. Remember that light waves refract because their speed changes when they move into a new medium. The different wavelengths of light travel at different speeds in a medium, such as the glass in a prism. Therefore, they bend by different amounts when they pass through the prism, as shown below. White light Red light 8. Identify Which color of light bends the most when it passes through the prism? Violet light Interactive Reader 368 Sound and Light
DISPERSION From longest to shortest wavelength, the colors in the visible spectrum are red, orange, yellow, green, blue, and violet. Violet light has the shortest wavelength and travels the most slowly through a medium, such as glass. Red light has the longest wavelength and travels the most quickly. Therefore, violet light bends more than red light when they pass from one medium to another. Thus, when white light passes from air into a glass prism, violet bends the most and red bends the least. The other colors are bent by an amount between violet and red. When the light exits the prism, the light is separated into the colors in the visible spectrum. The separation of light into different colors because of differences in wave speed is called dispersion. How Do Rainbows Form? Water droplets in the air can act like prisms. Sunlight that strikes a water droplet is dispersed into different colors as it passes from the air into the water. The refracted light rays strike the back surface of the water droplet at an angle. If the angle is small enough, the rays can reflect off the back of the droplet. Some of the light will then travel through the droplet. The light is dispersed again as it passes out of the water droplet back into the air. READING CHECK 9. Identify Which color of light travels the most slowly? READING CHECK 10. Define What is dispersion? Rainbows form through a combination of dispersion and reflection. 40º 42º R V 11. Describe What two effects combine to produce rainbows? V R When light finally leaves the droplet, violet light emerges at an angle of 40. Red light emerges at an angle of 42. The other colors of visible light are in between these angles. We see light from many droplets as arcs of color, which form a rainbow. Interactive Reader 369 Sound and Light
Section 4 Review SECTION VOCABULARY dispersion in optics, the process of separating a wave (such as white light) of different frequencies into its individual component waves (the different colors) lens a transparent object that refracts light waves such that they converge or diverge to create an image magnification the increase of an object s apparent size by using lenses or mirrors prism in optics, a system that consists of two or more plane surfaces of a transparent solid at an angle with each other 1. Identify Relationships How is dispersion related to refraction? 2. Describe Fill in the blanks in the flowchart below to describe how the human eye works. Light enters the eye through the, which refracts the light. Light passes through the pupil and is refracted again by the. The focuses the light on the back surface of the eye, the. The carries the signals to the brain, which interprets them as images. and in the retina detect the light and convert it to nerve signals. 3. List Give three examples of objects that use converging lenses. 4. Predict Consequences If all wavelengths of light traveled at the same speed in different mediums, would rainbows occur? Explain your answer. 5. Identify Which type of lens can produce both real and virtual images? 6. Apply Concepts Green light travels more slowly in a medium than yellow light. Which color of light is bent more when it passes through a prism? Interactive Reader 370 Sound and Light