LIGHT AND COLOR I. INTRODUCTION. A. Overview of the Unit

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1 1 LIGHT AND COLOR I. INTRODUCTION A. Overview of the Unit The unit on light and color has two major teaching and learning goals. The content goal is to help students come to understand and apply conceptual models to explain a wide range of observable phenomena in the domains of light and color. The metacognitive goal is to help students become more aware of and to take on more responsibility for their own learning (and to become more aware of others' thinking). Our materials for this unit are organized around a special set of instructional and learning activities to promote conceptual understanding. One of the most important activities are the instructor-led Demonstration and Discussion Activities. Their purpose is to elicit students' prior knowledge and to promote change in those ideas where it seems appropriate. The instructor demonstrates some phenomenon and generally asks students to make a prediction about what will happen if some change is made in the physical setup. At their seats the students discuss their ideas with their neighbors, sometimes writing down their predictions and drawing supporting diagrams. After a reasonable waiting period the instructor asks for a sample of ideas and helps clarify the students' reasoning. Then the instructor demonstrates the actual outcome of the task. Often (but not always) the result is a surprise to many students and suggests that they may want to consider changing the way they think about the phenomenon. At this point the instructor may elicit further discussion about the implication of the demonstration, or just make some suggestions. In any case, the purpose of the task is to begin the process of helping to move the student's thinking along so it may become closer to one of the target ideas, which we refer to as powerful ideas. The set of these powerful ideas comprise the conceptual models we are promoting in this unit. These demonstration and discussion activities could be carried out either in a large group assembly, or in smaller classrooms. The important point is to give students an opportunity to express their own ideas and to listen to those of others. A second type of activity suitable for either a large or small group setting are the Seat Experiments, in which small groups of students (we suggest three) explore new phenomena and apply powerful ideas. These experiments typically should last only a few minutes and are performed using simple apparatus that students might bring from home or could be provided by the instructor. (The students could divide up amongst themselves the Instructor Materials 2001 American Association of Physics Teachers

2 2 responsibilities for bringing in various items. The instructor would then need to have some items available in case students are absent.) The Lab Activities provide students with an opportunity to explore a given phenomenon. The labs are guided in the sense that all students are asked to make similar observations, so that there can be a common ground for discussion. The observations made in these labs form the basis for further discussion and development of some powerful ideas. Most of the Homework assignments give students the opportunity to apply their evolving knowledge in new contexts. Most assignments will ask students to write brief explanations and to draw diagrams. Finally, we provide a full set of Focus on Science materials. These summarize the powerful ideas. Students are assigned to read these after the ideas have been developed in class. In our approach for this unit, we have also included a number of suggested activities aimed at achieving the metacognitive goals. The demonstration and discussion activities and the various types of experimental activities provide extensive opportunities for small group and large group discussions where students can offer, examine, and critique their own ideas and those of their peers. This requires and should promote active participation of the students. We also suggest that students be given the opportunity of using the last five minutes of each discussion or lab session to reflect on their learning by writing a journal entry. (This is also discussed in a separate commentary.) In trying to design this unit around constructivist and conceptual change strategies, we have been confronted with a tension between letting students use and develop their own ideas and promoting a set of ideas that are consistent with those accepted and used in the scientific community. The approach we have decided to use is to initiate new topics by providing many opportunities for students to express their own ideas, provide situations in which those ideas are challenged, promote a set of powerful ideas (originating out of students' suggestions, but introduced by the instructor if necessary), and providing many opportunities for the students to apply these new ideas in motivational settings. The conceptual models we promote in the domains of geometrical optics and color and vision consist of a set of defining terms and verbal and diagrammatic ideas. Although these models are more qualitative and less rigorous than the models currently accepted by the scientific community, they have carefully been constructed to make sense to the students, to be expected to emerge out of discussion, and to enable the students to account for a wide range of phenomena American Association of Physics Teachers Instructor Materials

3 3 B. Acknowledgments and Origins of Ideas The activities for were, for the most part, developed by Fred Goldberg and associates at San Diego State University to be used in a course for prospective elementary education majors. We would also like to acknowledge the work done by Lillian McDermott and colleagues at the University of Washington, especially in the activities related to light. The color materials utilize ideas and activities drawn from Operation Physics. C. General Safety Considerations All activities in this unit are considered to be inherently safe in that no hazardous materials are used. However, one should use the customary caution when working with light bulbs to avoid burns or shocks. II. STUDENTS' NOTIONS ABOUT LIGHT AND COLOR A. Students' Prior Beliefs as Described in the Research on Student Conceptions Many of the so-called prior beliefs, or naive conceptions, associated with light and color are held by adults as well as young children, depending on their prior experiences. The naive conceptions listed below represent a partial list that focuses on those most likely to be encountered in the target audience. 1. Light is associated only with a source or its effects. Light is not considered to exist independently in space, and hence light is not considered as "traveling." 2. An object is seen because light shines on it. Light is a necessary condition for seeing, but there is no recognition of anything that moves between the object and the eye. 3. Lines drawn outward from a light bulb represent the "glow" surrounding the bulb. 4. Light travels only in a single direction from each point on a source. 5. Shaped sources will form shaped images even without a lens or mirror. 6. If light from a source passes through a pinhole to a screen, only a spot of light, the shape of the pinhole, will be observed on the screen. Instructor Materials 2001 American Association of Physics Teachers

4 4 7. For an observer to see the mirror image of an object, either the object must be directly in front of the mirror or, if not directly in front, then the object must be along the observer's line of sight to the mirror. The position of the observer is not important in determining whether the mirror image can be seen. 8. An observer can see more of his or her image by moving farther back from the mirror. 9. The mirror image of an object is located on the surface of the mirror. The image is often thought of as a picture on a flat surface. 10. The way a mirror works is the following: The image first goes from the object to the mirror surface. Then the observer sees the image on the mirror surface or the image reflects off the mirror and goes to the observer's eye. In other words, the image itself travels through space. 11. Light is reflected from smooth mirror surfaces but not from nonshiny surfaces. 12. Light always passes straight through a transparent material without changing direction. 13. When an object is viewed through a transparent solid or liquid material, the object is seen exactly where it is located. 14. Students often think of how a lens forms an image of a self-luminous object in the following way: A "potential image" which carries information about the object leaves the object and travels through space to the lens. When passing through the lens the "potential image" is turned upside down and may be changed in size. 15. When sketching a diagram to show how a lens forms an image, only those light rays are drawn which leave the object in straight, parallel lines. 16. Blocking a part of the lens surface would block the corresponding part of the image. 17. The purpose of the screen is to capture the image so it can be seen. The screen is necessary for the image to be formed. Without a screen, there is no image American Association of Physics Teachers Instructor Materials

5 5 18. An image can be seen on the screen regardless of where the screen is placed relative to a lens. To see a larger image on the screen, the screen should be moved farther back. 19. The pupil of the eye is a black spot or object on the surface of the eye. 20. The eye receives upright images. 21. The lens is the only part of the eye responsible for focusing light. 22. A white light source, such as an incandescent bulb or fluorescent bulb, produces light made up of only one color. 23. When white light passes through a prism, color is added to the light. 24. The rules for mixing color paints and crayons are the same as the rules for mixing colored lights. 25. The primary colors for mixing colored lights are red, yellow, and blue. 26. A colored light striking an object produces a shadow that is the same color as the light. 27. The shades of gray in a black and white newspaper photo are produced by using inks with different shades of gray. 28. When light passes through a colored filter, the filter adds color to the light. 29. The different colors appearing in pictures printed in magazines and newspapers are produced by using different inks with all the corresponding colors. 30. Color is a property of an object and is independent of both the illuminating light and the receiver (the eye). 31. White light is colorless and clear, enabling one to see the "true" color of an object. 32. When a colored light illuminates a colored object, the color of the light mixes with the color of the object. Instructor Materials 2001 American Association of Physics Teachers

6 6 B. Conceptions that Students Should Develop The conceptions that students should develop as they proceed through this unit are based on a set of so-called powerful ideas, which aid the student in the understanding of the various models. These ideas are listed below. 1. Light Emission from a Source From each point on a source light travels outward in all directions. 2. Traveling in Straight Lines Light travels in straight lines until it strikes a surface. 3. Reproduction of a Source To produce a pattern of light that is a reproduction of a source, light at each point in the pattern must have originated from only one corresponding point on the source. 4. Reflection of Light Light reflects from a surface according to the Law of Reflection: the angle of reflection equals the angle of incidence. 5. Refraction Light changes direction when traveling from one transparent medium into another, a process called refraction. When traveling from air into a solid or liquid material, the light bends towards the normal line by approximately one-third the angle between the initial direction line and the normal line. When traveling from a solid or liquid material into air, the light bends away from the normal line by approximately one-half the angle between the normal line and the initial direction line. 6. Real Image A real image point is formed when light: (1) diverges from an object point and (2) is redirected by an optical device to converge to another point in space. A real image is a collection of all the real image points. 7. Eye as an Optical Instrument The eye is an optical instrument whose purpose is to form sharp images of external objects on its screen (retina). The major functional parts of the eye are the cornea, iris, lens, and retina American Association of Physics Teachers Instructor Materials

7 7 8. Seeing an Object In order to see an object, light must diverge from that object and enter the eye of the observer. 9. Retina contains Light-Sensitive Regions The central portion of the retina consists of a large number of tiny lightsensitive regions. When light strikes a region, an electrical signal is sent to the brain whose strength increases with the intensity of the light. 10. Virtual Image A virtual image point is formed when light: (1) diverges from an object point; (2) is redirected by an optical device so as to appear to diverge from another point in space and then (3) enters an observer's eye who perceives the image to be located at that other point. 11. White Light Spectrum White light can be separated into a spectrum of colors that can be classified into three broad regions: red, green, and blue. 12. R-, G-, and B-Type Cones Each light-sensitive region on the retina contains three types of cone cells: one sensitive mainly to light from the red region of the spectrum (R-type), one sensitive mainly to light from the green part of the spectrum (G-type), and one sensitive mainly to the light from the blue region (B-type). When a cone cell receives light from its part of the spectrum, it sends an electrical signal to the brain, the strength of the signal increasing with the intensity of the light it receives. The cell continues sending its signal for about 1/30th of a second after the light stops. With prolonged illumination, the cell's signal diminishes and remains diminished for an average period of several seconds, after which the signal can return to its original strength. 13. Color Perception The perception of color depends on the relative strengths of the signals from the R-type, the G-type, and the B-type cones within each lightsensitive region. The relative strengths follow the rules of color addition. 14. Color Filter A color filter subtracts light from one or more regions of the spectrum. Paints, inks, dyes, and crayons behave like color filters. Instructor Materials 2001 American Association of Physics Teachers

8 8 III. COGNITIVE RATIONALE General Comments One of the major goals of the unit is to help the students develop a set of robust, meaningful, and valid ideas involving light and color phenomena. By the end of the unit, students who encounter a novel phenomenon should be able to account for it in terms of ideas that are closely aligned with those developed by the scientific community and which appear in textbooks. We believe it is reasonable within the hours of instruction for students to come to see these ideas as intelligible, plausible, and fruitful rather than abstract, nonsensical, and not very useful. Indeed, they should see them as a set of meaningful ideas that they themselves played a role in developing as the ideas emerged through discussion, rather than as dictums from authority. Students come into this unit with a set of notions about light and color notions that arise out of their attempts to make sense of everyday experience. It is the goal of this unit to have students examine their initial ideas in light of teacherdirected questions and, as a result of recognizing that some of their current ideas are not helpful in accounting for certain observed phenomena, to help them develop a new set of ideas whose development is directed so that what emerges are the so-called powerful ideas, which have both verbal and diagrammatic representations. Diagrams are powerful tools to guide thinking, and we want students to see the power of diagrams. These powerful ideas are representative of ideas held by the scientific community; however, they should emerge as reasonable and meaningful ways to account for various phenomena. The form of these ideas should be in terms meaningful to students. Their fruitfulness should emerge as students apply them to account for the myriad of different phenomena in light and color. In establishing criteria for pursuing a particular set of ideas (the list is not unique) as the goal ideas for the unit, we first wanted each idea to be represented both verbally and diagrammatically and for students to be able to switch easily between the two representations. Second, we intended for students to construct in their own words an equivalent formulation of each idea through classroom activities. Third, we intended the list to be robust and valid, i.e. students should be able to apply these ideas to account for a wide range of optical phenomena and make qualitative predictions about the behavior of simple optical systems. Fourth, we intended a formulation of these ideas which would not only make sense to students in terms of language, but would also be relatively easy to apply in practice. Thus, most of the ideas are stated in qualitative (and in some cases, approximate) form. Finally, we chose certain ideas because we know that they are crucial in understanding image formation and because they were often found to be lacking in the knowledge of post-instruction students American Association of Physics Teachers Instructor Materials

9 9 Specific Comments Investigation L1: Light and Illumination In working through these activities students should be able to develop and gather evidence to support three important powerful ideas: Light Emission from a Source, Light Traveling in Straight Lines, and Reproduction of a Source. Furthermore, they will learn how to draw ray diagrams to represent the behavior of light. We begin by asking students to describe how light leaves a source and then lead them though considerations of simple and complex shadows and pinhole phenomena. Investigation L2: Reflection of Light In these activities students will develop the Reflection powerful idea and will consider reflection of light from both mirrored (regular) and nonmirrored surfaces (diffuse reflection). They will also design devices to control light by reflection. The subject of mirror images will be deferred until Investigation L5. Investigation L3: Refraction and Real Images In this investigation students begin by considering what a lens does, then develop a set of rules to describe the refraction of light (the Refraction powerful idea). They learn that a triangular prism can be used as a simple model of a converging lens. The last activity focuses on a comprehensive comparison of the pinhole and converging lens systems and leads to the development of the Real Image powerful idea. Investigation L4: The Eye and Achromatic Vision In the first activity students investigate several properties of their eye and vision. These observations are then used as evidence to construct a simplified model of the eye which can account for achromatic vision (black and white and shades of gray only). Three powerful ideas are developed: Eye as an Optical Instrument, Seeing an Object, and Retina Contains Light-Sensitive Regions. Then students explore the nature of two common vision defects (farsighted-ness and nearsightedness) and infer their optical correction. In the last activity the students study how black and white newspaper pictures are printed and apply the achromatic model of vision to explain the phenomenon. Instructor Materials 2001 American Association of Physics Teachers

10 10 Investigation L5: Images that Cannot be Formed on Screens The concept of a virtual image is probably the most abstract of all the ideas to be developed in the unit. This investigation begins by having students develop the idea that when looking at their image in a mirror their eyes are not focusing on the mirror surface but at some point beyond. In the following two activities the students use a half-silvered mirror to develop an understanding of how a mirror works (and develop the Virtual Image powerful idea) and then apply it to design a magical illusion involving the optical transformation of one object into another. The next activity also involves mirrors and is a real puzzler. Finally, the students explore virtual images involving refraction. Investigation L6: Color Addition and Color Vision Color phenomena are intrinsically interesting to students. In this investigation they will explore various phenomena involving additive color mixing and will use their observations as empirical support for a model of color vision. They will come to understand the three powerful ideas: White Light Spectrum, R-, G-, and B- Type Cones, and Color Perception. Most of the students' prior experience with color mixing consists of mixing color paints and crayons and involves processes that are different from those explored in the current investigation. This may be a source of difficulty for some students and will need to be addressed carefully. The mixing of paints, inks, and dyes will be discussed in Investigation L7. Investigation L7: Color Filters The idea of color filters form the basis for understanding the processes involved in mixing color paints, dyes, and inks. Rather than give this process a special name (color subtraction), we prefer to have students use both the Color Filter powerful idea along with the ideas involving additive color mixing to interpret and explain these phenomena American Association of Physics Teachers Instructor Materials

11 11 IV. INSTRUCTOR NOTES A. Equipment List Equipment for Instructor Demonstrations 1. 1 each 100 W and 150 W frosted bulbs mounted in sockets with electrical cords Clear bulb with U-shaped filament (40 W appliance bulb) mounted in socket with electrical cord 2 high-intensity point sources or 2 clear bulbs with linear filaments (showcase bulbs) mounted in sockets with power cords 2. 3 slide projectors 3 variacs (to connect to slide projectors) 3 circular slit slides (see Activity L6.1) 3. VCR and 1, or preferably 2, television monitor(s) Videotape showing color bars (red, green, blue, white, yellow, cyan, magenta, and black) or still picture with many broad areas of color 4. Blackboard optics setup with: source(s) that produce 1, 2, or 3 narrow beams; source that produces single very broad beam (if possible, but not necessary); concave mirror; solid semicircular block; solid triangular prism; two converging lenses; a diverging lens 5. Holographic diffraction grating and cardboard with long narrow slot to place on stage of overhead projector (and which covers entire stage); or a good quality diffraction grating or prism, and a 35-mm size aluminum slide with a narrow slot in it (see Activity L6.1) in. x 12 in. flat mirror (a mirror tile would be ideal) with means to mount it upright on table and a cardboard cover for the mirror Small plane mirror 7. Lens setup; consisting of a frosted light bulb or other self-luminous object, a converging lens, and a translucent screen all mounted in holders on a table or optical bench 8. Some pairs of corrective eyeglasses for nearsightedness and farsightedness (These can be borrowed from students. Have some lens paper available so students can "feel" the shapes of these corrective lenses without getting them too dirty.) 9. Set of 2 in. x 2 in. (or larger) colored filters: red, green, blue, yellow, magenta, and cyan 10. Colorful 35-mm slide (to project an interesting image) Long white rod (approximately 0. 5 m) Masking tape Blank white index cards Miscellaneous pieces of cardboard Nail Extension cord Instructor Materials 2001 American Association of Physics Teachers

12 12 For Each Group of Students Black magic markers Bench clamps, rods, supports, etc. to mount mirrors Cardboard (a square of black poster board, about cm on a side) Clay (small ball 1 cm in diameter) Color filters: set of 2 in. x 2 in. (approximate size) red, green, blue, cyan, yellow, and magenta filters Color keys, if available see Activity L7.2 Colored squares: set of 2 in. x 2 in. squares of bright colors red, blue, yellow, and green (fluorescent colors would be good) Converging lens (focal length of 10 cm and 15 cm) and upright mount Crayons or colored markers Dark marker pen 2 flashlights Half-silvered (one-way) small mirror (available from Edmund Scientific) Index cards blank white Light source frosted bulb in socket (40, 60 or 75 W) Light source frosted bulb in socket (100 W to 150 W) Light source point source of light (e.g., a flashlight bulb powered by either 2 batteries or a low voltage power supply) or a clear bulb with a linear filament (showcase bulb) mounted in base with power cord Light sources 3 "point" sources needed flashlight bulbs in holders with batteries would be suitable; see Activity L6.2 Light sources 2 devices to produce a narrow beam of light. If a beam projector is not available, one could make such a device from a flashlight. Use a piece of cardboard or black construction paper and cut out a circular disc that would fit into the head of the flashlight. Then, with a razor or other sharp instrument, cut a narrow slot in the middle of the disc. Mount the disc inside the head of the flashlight. This should produce a reasonably narrow beam that could be seen against a piece of white paper. Magnifier (8-12 cm converging lens would be fine) Manilla envelope containing several 1 in. x 1 in. squares of different colors (red, blue, green, yellow, orange, etc.) Manilla file folder Masking tape Mirror, small flat one (1 for each student) Mirror tile: 12 in. x 12 in. Mirrors flat, several of different sizes Nail with sharp point Paper black (8 1/2 in. x 11 in.) Paper white (8 1/2 in. x 11 in.) Piece of dark poster board (approximately 4-in. square) Protractor Ruler meterstick Ruler small cm Scissors Screen (translucent, roughly 10 cm square); (approximately 12 in. x 12 in. white cardboard) Screen translucent (tracing paper mounted on a cardboard frame is fine) Straw (drinking) Styrofoam cups (2) Transparent solid rectangular block (approximately 1 in. x 1 in. x 2.5 in.) Wood blocks, miscellaneous sizes (or books) 2001 American Association of Physics Teachers Instructor Materials