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13.3 Images in Lenses OVERALL EXPECTATIONS Time • demonstrate scientific investigation skills in the four areas of skills • demonstrate an understanding of various characteristics and properties of light, particularly with respect to reflection in mirrors and reflection and refraction in lenses 45–60 min Vocabulary • emergent ray Skills SPECIFIC EXPECTATIONS Scientific Investigation Skills • conduct inquiries, controlling some variables, adapting or extending procedures as required, and using standard equipment and materials safely, accurately, and effectively, to collect observations and data • analyze and interpret qualitative and/or quantitative data to determine whether the evidence supports or refutes the initial prediction or hypothesis, identifying possible sources of error, bias, or uncertainty • draw conclusions based on inquiry results and research findings, and justify their conclusions Predicting Performing Observing Analyzing Equipment and Materials per group: • ray box • single-slit mask • rectangular prism • blank sheet of paper Assessment Resources Understanding Basic Concepts • explain the conditions required for partial reflection/refraction and for total internal reflection in lenses, and describe the reflection/refraction using labelled ray diagrams • describe the characteristics and positions of images formed by converging lenses, with the aid of ray diagrams • identify ways in which the properties of mirrors and lenses determine their use in optical instruments • identify the factors, in qualitative and quantitative terms, that affect the refraction of light as it passes from one medium to another KEY CONCEPTS • A lens is a transparent object used to change the path of light. Assessment Rubric 2: Thinking and Investigation Assessment Summary 2: Thinking and Investigation Other Program Resources BLM 13.3-1 Imaging Properties of Lenses Skills Handbook 1. Safe Science Skills Handbook 3. Scientific Inquiry Skills Science Perspectives 10 website www.nelson.com /sciencepersectives/10 • Parallel light rays are refracted through a focus when they pass through a converging lens. • Geometric optics can be used to determine the path of light rays through lenses. Related Resources Gizmos: Basic Prism EVIDENCE OF LEARNING Look for evidence that students can • explain the three rules for imaging • predict image characteristics for a light source at various distances from the lens • understand how virtual images are formed in diverging lenses or, for converging lenses, when the light source is located between the prime focal point and the lens Hakim, Joy. The Story of Science, Newton at the Center. Smithsonian Books, 2005. Science Perspectives 10 ExamView® Test Bank Science Perspectives 10 Teacher eSource SUITE Upgrade Science Perspectives 10 website www.nelson.com /sciencepersectives/10 NEL 55308_04_ch13_p875-938 pp3.indd 887 Chapter 13 Lenses and Optical Devices 887 11/20/09 7:01:10 PM ▼ SCIENCE BACKGROUND Prisms are displaced sideways. The parallelism of the rays only occurs because of the parallelism of the prism. For a prism of a different shape, the incident and emergent rays would not be parallel. • A prism is a transparent optical device that has polished, flat surfaces that can be used to either reflect or refract light. The precise angles of any prism, as well as the prism’s composition, depend on the intended use. Most prisms are made of glass, and their size and thickness can vary greatly according to their function. The traditional geometric prism is triangular, with its base and both sides forming triangles. Prisms, however, can take many geometric forms. • There are three imaging rules for converging lenses: 1) A ray parallel to the principal axis is refracted through the principal focus. 2) A ray through the secondary principal focus is refracted parallel to the principal axis. 3) A ray through the optical centre continues straight through without being refracted. • A prism is transparent to the wavelengths of light it is designed to affect. When refracting light, prisms can redirect white light or break light into its spectral colours. For refraction, the various wavelengths of light are slowed differently as they pass through a prism, producing the colours of the spectrum. • The rules for locating images in diverging lenses are the same as for converging lenses, except that the light rays only appear to come from the principal focus; they do not really. Once students master the rules for converging lenses, dealing with diverging lenses will be easier. • When light passes through a rectangular prism, it is refracted upon entering (at the air–glass boundary) and then upon emerging (at the glass– air boundary). The two refractions produce emergent rays that are parallel to the incident rays, but which • The characteristics of the images produced by a converging lens depend on the location of the object relative to the secondary principal focus. A diverging lens always produces the same image characteristics no matter where the object is. POSSIBLE MISCONCEPTIONS Identify • Students may presume that as you move toward a convex lens, the real image keeps getting larger. Clarify • When the object is located exactly at F´, no image appears at all. Locating the object closer than F´ results in an image that gets larger, but it is no longer a real image. For any distance less than F´ the image is virtual. Ask What They Think Now • At the end of the section ask, What kind of image will you see when the object distance is less than F´? Students should recognize that they will see a virtual image that gets larger. 888 Unit E: Light and Geometric Optics 55308_04_ch13_p875-938 pp3.indd 888 NEL 11/20/09 7:01:10 PM TEACHING NOTES Writing Tip Engage • On the board, draw Figure 3(a) from page 558 of the Student Book but do not include the light rays. Ask the following questions, allowing time for students to speculate about each response: Do you think an image will be produced by this lens? If so, where will the image appear? (between F and 2F ) Will the image be larger or smaller than the object? (smaller) Will the image be upright or inverted? (inverted) After you have revealed the answers, tell students that they will learn how to form images for both convex and concave lenses in this section. Explore and Explain • Have students complete Try This: Exploring the Rectangular Prism. This activity gives students an opportunity to explore the relationship between rays incident to and emergent from a prism. An understanding of this relationship is necessary for students’ understanding of how to draw ray diagrams for lenses. T RY THIS Writing a Critical Analysis Suggest students consider what position they would take in a critical analysis of a camera with a fixed focal length lens. Have them write this in their notebooks and include an outline of the reasoning they would use to support their position. EXPLORING THE RECTANGULAR PRISM Skills • Predicting, Performing, Observing, Analyzing Purpose • Students will observe the effect of a prism, particularly its thickness, on an incident light ray. Equipment and Materials (per group): ray box; single-slit mask; rectangular prism; blank sheet of paper Student Safety • Have students take care when handling prisms, which may have a sharp edge if they have been chipped during use. • Remind students not to remove the plug from the ray box by pulling on the cord, as it might damage the wire. Notes • Students may have to press down on the prism to prevent light rays from travelling under the prism and confusing the results. • Have students work in pairs for this activity. Suggested Answers A. The emergent ray was parallel to the incident ray but displaced sideways. B. The sideways displacement of the emergent ray was smaller. C. If I had used an extremely thin rectangular prism, the two rays would very nearly lie on a single line. D. The results should confirm the answer to C. • Go over the rules for converging lenses in Figure 2 on page 557 of the Student Book. Again, draw the lens, O, 2F´, F´, F, 2F, and the principal axis without including any of the light rays shown in the diagram. Then have a volunteer read the first rule: A ray parallel to the principal axis is refracted through the principal focus, F. Draw a parallel incoming ray. Ask, What path will it take on the other side of the lens? Show that the ray passes through F. Repeat the process for rules 2 and 3, showing light rays that pass through O and F´. • Re-draw the Figure 3(a) set-up you drew for the Engage section. Use rules 1–3 to draw three light rays and show where they intersect. Have a volunteer draw in the image. Ask, Why is the image real? (It is formed by actual light rays.) Why is the image inverted? (The rays intersect below the principal axis.) Why is the image smaller than the object? (The distance between the intersecting rays and the principal axis is shorter than the distance between the axis and the object.) NEL 55308_04_ch13_p875-938 pp3.indd 889 Chapter 13 Lenses and Optical Devices 889 11/20/09 7:01:10 PM Reading Tip Making Connections Have students locate the three versions of reflections in a concave mirror shown in Figure 6 of Section 11.9 on page 497 of the Student Book. Allow them a few minutes to examine the illustrations. Ask volunteers to point out similarities and differences between the illustrations in Figure 6 of Section 11.9 and Figure 3 of this section. Students should note that the rays shown passing through F for concave mirrors and F ´ for converging lenses are always parallel to the principal axis after either reflecting or refracting. Unit Task B ookmark Guide students in determining how image characteristics will come into play in their device. Even if students have not decided which device to build, they can consider the fact that the final image cannot be inverted if it is to be used by humans. Also, have students note that where the image appears will greatly affect the construction of their device. CHECK • Go over Figures 3, 4, and 5 on pages 558 and 559 and make sure that students understand each part, as various incident rays behave differently in other parts of the illustration. Ask students questions such as, Is this incident ray parallel or angled? Where is the light source now? What happens when it emerges and why? • As you work through the illustrations with students, ask them to note the size of the image, its attitude or orientation, and if it is real. Virtual images are drawn with dashed lines around them. Emphasize that the rules and characteristics discussed so far are only for converging lenses. • Explain to students that the rules for locating images in diverging lenses are the same as for converging lenses, except for the fact that the light rays do not come from the primary focus. • Copy Figures 6 and 7 on pages 559 and 560 of the Student Book on the board or overhead. Have students use their pencil to trace the rays in each of the figures to get practice locating the image in a diverging lens. Use the figures you drew on the board or overhead to demonstrate the rules for locating the images. Discuss any issues or questions that students have. Ask students to state the characteristics of images produced by diverging lenses. Students should be able to state that the image is always upright, virtual, and on the same side of the lens as the object. Extend and Assess • Have students restate in their own words the rules they have learned about converging and diverging lenses. They should combine observations of the position of the light source and the characteristics of the image produced with the three imaging rules. • Have students imagine that rule 1 for converging lenses is being questioned. Ask, How could you prove that rule 1 is true? Have students suggest an experiment to show that rule 1 is true. The experiment should involve shining light beams parallel to the principal axis showing that they all pass through a common point, F. Repeat this for diverging lenses. • Have students complete BLM 13.3-1 Imaging Properties of Lenses, in which they will complete diagrams that demonstrate object-image relationships of lenses described in this section. • Have students complete the Check Your Learning questions on page 561 of the Student Book. YOUR LEARNING Suggested Answers 1. (a) Rays parallel to the principal axis go through F on the other side. Rays through F´ run parallel to the principal axis. Rays through O keep going straight. (b) For a diverging lens, the only difference is that the rays do not actually come from the principal focus; they just appear to. 2. (a) (i) One ray from the tip of the candle flame goes parallel to the principal axis and refracts through F. A second ray from the flame tip goes unbent through O. Both rays meet below the principal axis at the image of the flame tip. (ii) One ray from the tip of the object goes parallel to the principal axis and refracts through F. A second ray from the arrow point goes unbent through O. Both rays meet above the principal axis at the image of the arrow point. (iii) One ray from the tip of the candle flame goes parallel to the principal axis and refracts through F. A second ray from the flame tip goes unbent through O. These rays diverge and must be traced back to the image of the flame tip on the same side of the principal axis as the candle. 890 Unit E: Light and Geometric Optics 55308_04_ch13_p875-938 pp3.indd 890 NEL 11/20/09 7:01:10 PM (iv) One ray from the object arrow point goes parallel to the principal axis and refracts aligned with F. A second ray goes unbent through O. These rays diverge and must be traced back to the image of the arrow point on the same side of the principal axis as the object arrow. (b) (i) smaller than the object, inverted, located between 2F and F, real image object (ii) larger than the object, inverted, located beyond 2F, real 2F ⬘ F⬘ F 2F object 2F ⬘ F⬘ F 2F image (iii) larger than the object, upright, located on same side, virtual 2F ⬘ F⬘ image object (iv) smaller than the object, upright, located on the same side, virtual F 2F object 2F 3. A ray from the object arrow tip going parallel to the principal axis will refract through F and pass through the image of the arrow tip. This ray crosses the principal axis at F. F F⬘ image 2F ⬘ object F image 4. (a) and (b) object (i) 2F ⬘ F⬘ F 2F image object image (ii) NEL 55308_04_ch13_p875-938 pp3.indd 891 2F ⬘ F⬘ F 2F Chapter 13 Lenses and Optical Devices 891 11/25/09 12:36:16 AM 5. A real image is never formed because the emergent rays from a divergent lens always spread apart. A real image can only be formed if the emergent refracted rays cross or converge. The human brain interprets diverging light from this type of lens as if it were coming from a single focus point, creating a virtual image. 6. The size of the virtual images produced by a converging mirror changes depending on the distance of the object. An image produced by a diverging mirror never changes in size. A converging lens produces a larger-than-life virtual image, and a diverging lens produces a smaller-than-life virtual image. 7. If a lens produces a virtual image, it will be upright and located on the same side of the lens as the object. A real image is produced if the orientation of the image is inverted to that of the original object. 8. (a) A converging lens—a real image is produced on the screen. (b) The diagram should show the object (the film) below the principal axis, located between 2F´ and F´, so that an enlarged, right side up image is produced. movie screen film source 2F F F 2F (c) The image is larger than life, inverted, beyond 2F, and real; for the image to appear right side up, the film must be inserted in the projector upside down. DIFFERENTIATED INSTRUCTION Business Studies Connections Encourage students to research developments and innovations related to lenses and how those changes have affected people’s lives. Have them relate these developments to demand in the market and the efforts of entrepreneurs and established companies to meet these demands. For example, have them consider the evolution of the camera. 892 • Encourage visual/spatial students to create a poster for the class showing the information in Table 1 on page 559 of the Student Book with diagrams, as well as information on diverging lenses. Encourage students to refer to the table as they go over different situations in this section and other sections of the chapter. • Bodily/kinesthetic learners should benefit from returning to the lens, candle, and screen set-up of the previous section’s activity. Allow students to conduct the same experiment as before, but this time using the illustrations and descriptions contained in this section. Students should try to duplicate the images and demonstrate the different movements and relocation of the light, explaining as they do the experiments how the principles of refraction are producing what they see. ENGLISH LANGUAGE LEARNERS • Pair students with widely varied comprehension, vocabulary, or fluency strengths. Have students model reading for each other. Students can work to define terminology in their own words and guide each other through their reading. Unit E: Light and Geometric Optics 55308_04_ch13_p875-938 pp3.indd 892 NEL 11/20/09 7:01:11 PM