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Lenses Form Images by Refracting Light

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Lenses Form Images by Refracting Light KEY CONCEPT Lenses form images by refracting light. BEFORE, you learned NOW, you will learn •Waves can refract when • How a material medium can they move from one refract light medium to another • How lenses control refraction • Refraction changes the • How lenses produce images direction of a wave VOCABULARY EXPLORE Refraction lens p. 601 How does material bend light? focal length p. 603 PROCEDURE MATERIALS • clear plastic cup 1 Place the pencil in the cup, as shown in • pencil the photograph. Look at the cup from the •water side so that you see part of the pencil •mineral oil through the cup. 2 Fill the cup one-third full with water and repeat your observations. 3 Gently add oil until the cup is two-thirds full. After the oil settles into a separate layer, observe. WHAT DO YOU THINK? • How did the appearance of the pencil change when you added the water? the oil? • What might explain these changes? A medium can refract light. When sunlight strikes a window, some of the light rays reflect off the surface of the glass. Other rays continue through the glass, but their direction is slightly changed. This slight change in direction is called refraction. Refraction occurs when a wave strikes a new medium—such as the window—at an angle other than 90° and keeps going forward in a slightly different direction. Refraction occurs because one side of the wave reaches the new medium slightly before the other side does. That side changes speed, while the other continues at its previous speed, causing the wave to turn. Check Your Reading How does the motion of a light wave change when it refracts? Chapter 18: Light and Optics 599 Refraction of Light Recall that waves travel at different speeds in different mediums. COMBINATION NOTES The direction in which a light wave turns depends on whether the Sketch the ways light is new medium slows the wave down or allows it to travel faster. Like refracted when it moves into a denser medium and reflection, refraction is described in terms of an imaginary line—called into a thinner medium. the normal—that is perpendicular to the new surface. If the medium slows the wave, the wave will turn toward the normal. If the new medium lets the wave speed up, the wave will turn away from the normal. The wave in the diagram below turns toward the normal as it slows down in the new medium. air air (thin)glass (thin) (dense) 1 Waves moving at an angle into a denser medium turn toward the normal. 1 2 normal 2 Waves moving at an angle into a thinner light medium turn away wave from the normal. reading tip Light from the Sun travels toward Earth through the near vacuum A dense medium has more of outer space. Sunlight refracts when it reaches the new medium of mass in a given volume than Earth’s upper atmosphere. Earth’s upper atmosphere is relatively thin a thin medium. and refracts light only slightly. Denser materials, such as water and glass, refract light more. light By measuring the speed of light in different materials and compar- water ing this speed to the speed of light in a vacuum, scientists have been droplet able to determine exactly how different materials refract light. This knowledge has led to the ability to predict and control refraction, which is the basis of much optical technology. color Refraction and Rainbows spectrum Light passing through a droplet of water is You’ve seen rainbows in the sky after a rainstorm or refracted twice, forming hovering in the spray of a sprinkler. Rainbows are a color spectrum. caused by refraction and reflection of light through spherical water drops, which act as prisms. Just as a prism separates the colors of white light, producing the color spectrum, each water drop separates the wavelengths of sunlight to produce a spectrum. Only one color reaches your eye from each drop. Red appears at the top of a rainbow because it is coming from higher drops, while violet comes from lower drops. 600 Unit 4: Waves, Sound, and Light Shape determines how lenses form images. When you look at yourself in a flat mirror, you see your image clearly, without distortions. Similarly, when you look through a plain glass reading tip window, you can see what is on the other side clearly. Just as curved Distort means to change mirrors distort images, certain transparent mediums called lenses alter the shape of something by twisting or moving the what you see through them. A lens is a clear optical tool that refracts parts around. light. Different lenses refract light in different ways and form images useful for a variety of purposes. Convex and Concave Lenses Like mirrors, lenses can be convex or concave. A convex lens is curved outward; a concave lens is curved inward. A lens typically has two sides that are curved, as shown in the illustration below. Convex Lens Concave Lens focal point principal axis A convex lens causes parallel light A concave lens causes parallel light rays to meet at a focal point. rays to spread out. Convex Parallel light rays passing through a convex lens are refracted reminder inward. They meet at a focal point on the other side of the lens. The The focal point is the point at which parallel light rays rays are actually refracted twice—once upon entering the lens and meet after being reflected once upon leaving it. This is because both times they are entering a or refracted. new medium at an angle other than 90 degrees. Rays closest to the edges of the lens are refracted most. Rays passing through the center of the lens—along the principal axis, which connects the centers of the two curved surfaces—are not refracted at all. They pass through to the same focal point as all rays parallel to them. Concave Parallel light rays that pass through a concave lens are refracted outward. As with a convex lens, the rays are refracted twice. Rays closest to the edges of the lens are refracted most; rays at the very center of the lens pass straight through without being deflected. Because they are refracted away from each other, parallel light rays passing through a concave lens do not meet. Check Your Reading Compare what happens to parallel light rays striking a concave mirror with those striking a concave lens. Chapter 18: Light and Optics 601 How a Convex Lens Forms an Image A convex lens forms an image by refracting light rays. Light rays reflected from an object are refracted ray A when they enter the lens and again focal point when they leave the lens. They meet to form the image. focal lengths 1 Light rays reflect off the penguin in all directions, and many enter the lens. Here a single ray (A) from the top of the penguin enters the lens and is refracted downward. ray B point C 2 Another light ray (B) from the top of the point penguin passes through the lens at the bottom D and meets the first ray at point C. All of the rays from the top of the penguin passing through the lens meet at this point. 3 All of the light rays from the bottom of the penguin meet at a different point (D). Light rays from all parts of the penguin meet at corresponding points on the image. Where do light rays reflected from the middle of the penguin meet? 602 Unit 4: Waves, Sound, and Light Images Formed by Lenses When light rays from an object pass through a lens, an image of SIMULATION the object is formed. The type of image depends on the lens and, for CLASSZONE.COM convex lenses, on the distance between the lens and the object. Work with convex and concave lenses to form Notice the distance between the penguin and the lens in the images. illustration on page 602. The distance is measured in terms of a focal length, which is the distance from the center of the lens to the lens’s focal point. The penguin is more than two focal lengths from the camera lens, which means the image formed is upside down and smaller. If the penguin were between one and two focal lengths away from a convex lens, the image formed would be upside down and larger. Overhead projectors form this type of image, which is then turned right side up by a mirror and projected onto a screen for viewing. Finally, if an object is less than one focal length from a convex lens, it will appear right side up and larger. In order to enlarge an object so that you can see details, you hold a mag- nifying lens close to the object. In the photograph, you see a face enlarged by a magnifying lens. The boy’s face is less than one focal length from the lens. If you look at an object through a concave lens, you’ll see an image of the object that is right side up and smaller than the object normally appears. In the case of concave lenses, the distance between the object and the lens does not make a dif- ference in the type of image that is formed. In the next section you’ll see how the characteristics of the images formed by different lenses play a role in complex optical tools. When will an image formed by a convex lens be upside down? KEY CONCEPTS CRITICAL THINKING CHALLENGE 1.
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