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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
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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 Light and Color 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.
©2001 American Association of Physics Teachers
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B. Acknowledgments and Origins of Ideas
The activities for Light and Color 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.
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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.
©2001 American Association of Physics Teachers
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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.
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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.
©2001 American Association of Physics Teachers
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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.
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III. COGNITIVE RATIONALE
General Comments
One of the major goals of the Light and Color 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 16-20 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.
©2001 American Association of Physics Teachers
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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.
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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 Light and Color 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 BType 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.
©2001 American Association of Physics Teachers
Light and Color
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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)
6.
12 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
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For Each Group of Students
Black magic markers
Bench clamps, rods, supports, etc. to mount mirrors
Cardboard (a square of black poster board, about 10-12 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
Light and Color
Instructor Materials