D. Student Activities with Embedded Instructor Notes
Investigation L1: Light and Illumination
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Activity L1.1: How does light leave a bulb?
(Demonstration)
Instructor Note: 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 through considerations of simple and
complex shadows and pinhole phenomena.
Equipment: 100 W frosted bulb mounted in socket with electrical cord, clear bulb with
U-shaped filament (40 W appliance bulb) also mounted in socket with electrical cord
and with optional variac (to control brightness), extension cord, if necessary, white
screen.
This activity should help students come to realize that: (1) light leaving each point on a
source goes out in all directions (away from the source); and (2) a ray diagram
represents the behavior of light. Begin by showing a turned off frosted light bulb that is
mounted in a socket. Turn the bulb on. Mention the obvious, that the bulb is giving off
light. Then have them answer the following questions. Remind the students that they
are to think about the question and write down their own idea first. Then, they are to
discuss their idea and show their sketch to other members of the group.
1.
?
2.
??
WHAT IS YOUR IDEA? Your instructor will show you a lighted bulb. How
does the light leave the bulb? Draw a sketch to represent your idea.
WHAT ARE THE GROUP'S IDEAS? How did the other members of your
group represent the light leaving the bulb? Draw sketches that differ from
yours.
After several minutes you should walk around the room and glance at the
sketches, taking note (or sketching yourself) some of the ways that students
are representing light leaving the bulb. When you think the conversations
have subsided you should tell students that you will draw representative
sketches on the board. For each sketch, ask for a student who drew the sketch
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to explain briefly his or her idea. The collection of drawings should include at
least those shown in the following diagrams, plus perhaps others. If one of these
is missing, you might add it and say that last semester some students drew this
sketch. Obviously, as each of these is being discussed, you should refrain from
making value judgments, except to realize that each sketch is a valid
representation of a particular way of conceptualizing the situation.
We have found that students who draw a diagram like A are not necessarily
thinking about representing light going outward from the source. Instead, they
are representing the brightness (or intensity) surrounding the bulb. On the other
hand, students who draw diagrams similar to the others seem to be thinking of
representing light going outward from the bulb, but their representations are
different. Students might draw sketches B or C if they are representing light as
waves going out from the bulb. Sketches D and E are similar, except that the
latter diagram indicates that the lines have some direction associated with them.
Sketches C, D, and E have in common the representation that light seems to go
out in one direction from each point on the bulb. Students who draw a diagram
similar to sketch F, however, seem to be representing explicitly that light goes
out in many directions from a single point. (We have found that very few
students draw this at this stage.)
After you have represented the students' ideas on the board, you should
point out that one of the reasons for drawing such diagrams is to enable
people to describe conveniently how light is behaving in a particular
circumstance. One should be able to look at a diagram and interpret how
light is behaving. With this in mind you should go over each diagram again
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and interpret what each suggests about the behavior of light. Students should
realize that diagram A is probably not very useful because it doesn't suggest that
light leaves the source and goes out into the surrounding space (because of the
gaps). Diagrams B and C seem to emphasize that light travels outward from the
source as waves. Diagrams D, E, and F seem to suggest the direction that light is
traveling. At this point you might suggest that for the purpose of drawing
diagrams to represent how light behaves in many different circumstances it
would be more practical to use the diagram to show the direction that light is
traveling. In that sense, diagrams D, E, and F might be more helpful. There are
still differences, however, between these three. Diagrams D and E are similar
except that E has arrows. If we can agree that the diagram supposedly shows the
direction that light travels, then the arrows might be more useful. You could
now define a light ray as a line segment with an arrow that represents the
direction that light is traveling. We should aim to draw light rays in our
diagrams to represent how light behaves in various circumstances.
Finally, there still is a difference between sketch F and sketches D and E.
Sketches D and E seem to suggest that light travels outward in one direction
from each point, whereas sketch F suggests that light travels outward in many
directions from each point. Is this a significant difference? To help answer this
question you should move on to the next demonstration. (Do not erase the
diagrams yet.) Aim the clear bulb with U-shaped filament (henceforth called the
U-bulb) at the students and turn it on, showing them the U-shape of the filament.
(Since it is very bright, you might want to use a variac to cut down the brightness
when you aim it at the students. Also, if the students in the back of the room
can't see the bulb clearly, you might sketch the shape of the filament on the
overhead or chalkboard.) Then turn it off, hold it about 40 cm or so from the
screen, aim it towards the screen, and have the students address the first
question.
3.
WHAT IS YOUR IDEA? Your instructor will show you a special clear bulb
with a U-shaped filament. If it faced the screen and was turned on, what
would you observe on the screen? What is your reasoning?
?
4.
??
WHAT ARE THE GROUP'S IDEAS? Summarize ideas of the group that are
different from yours. If your group can reach consensus, describe what it is.
We have found that this task always elicits good discussion. Again walk
around the class and note what students are doing. After allowing several
minutes for discussion, you should elicit students' ideas. It has been our
experience that most students will predict a pattern of illumination on the
screen that conveys the U-shape of the filament. They differ in how sharp
they think the pattern will be. Some will suggest that the pattern is well
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defined and support this with a diagram showing individual light rays leaving
different points on the filament and going directly to the screen, along the
perpendicular. Other students realize that light will spread out from the bulb,
but suggest that, even with the spread out illumination, there will still be a
recognizable U-shaped pattern. Their diagrams show light spreading out
somewhat from the filament, but still going preferentially in a specific direction.
We have found very few students draw diagrams showing light leaving each
point on the filament and going out to all points on the screen, suggesting that
the screen would be fully illuminated.
After all ideas are represented and clarified, you should demonstrate what
actually happens. Darken the room lights (if possible) and turn on the bulb.
Most students are completely surprised by the outcome. The screen is fully (but
not uniformly) illuminated with the central region of the screen somewhat
brighter than the parts furthest away from the bulb, and with no pattern of
illumination that bears any resemblance to the U-shaped filament.
5.
MAKING OBSERVATIONS: What did you observe on the screen when the
special bulb was turned on? Did this confirm your prediction? If not, how
was it different from your prediction?
6.
MAKING SENSE: What does this last observation suggest to you about how
light leaves a source? Look back at your original idea in step one. Do you
believe it is now reasonable to think differently about how light leaves a
source? Which of the sketches suggested by your classmates seems to make
the most sense to you now?
➥
This discrepant event helps students recognize that their ideas about how light
leaves a source may need to be modified. Ask students to suggest ways of
accounting for this interesting demonstration. A student should suggest that the
observation of full-screen illumination provides evidence for the idea that light
goes out in all directions from each point on the source filament. (Mention that
we could consider the whole filament a sequence of source points.) In fact, the
particular shape of the filament should make no difference; the screen would still
be fully illuminated. Returning to the previous diagrams on the board, it
appears that Sketch F represents this idea, and we could improve on the diagram
by adding arrows. Suggest to students that this idea, which describes how light
leaves a source, is a very important one and that it can help guide their thinking
in many different circumstances.
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We will call such an idea a powerful idea, and this one can be referred to as the
Light Emission from a Source powerful idea. It can be represented
diagrammatically as seen below.
At this point it is useful to lead the students to the idea that to see an object, light
must travel from the object to the eye. Later in the Light and Color unit, we will
develop a powerful idea that incorporates this.
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Notes
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Activity L1.2: What are shadows?
(Demonstration/Laboratory)
Instructor Note: When drawing diagrams with light rays, we are assuming that light travels
in straight lines. Students may have mentioned this before, but the following activity will
provide evidence that this is a reasonable idea. In fact it will become the second powerful idea in
this unit. This can be done as a laboratory activity or carried out in a large-class setting.
Equipment: High-intensity point source or clear bulb with linear filament mounted in
base with power cord, rectangular shaped object, screen.
1.
WHAT IS YOUR IDEA? What is a shadow? What conditions are necessary for
a shadow to be formed?
?
2.
??
WHAT ARE THE GROUP'S IDEAS? Compare your idea about how a shadow
is formed with the ideas of others in your group. Record those that are
different from yours.
After students have discussed this for a few minutes, elicit a sampling of ideas,
help clarify them, but it may not be fruitful to have any extensive discussion
about the merits of the ideas at this point.
At the front of the room you should have a setup with a bulb and a large flat
rectangular object (a box would be suitable) mounted on a table. The screen
should be located so that if you were to turn on the bulb there would be a welldefined shadow on the screen. To insure the shadow will be sharp and
uniformly dark, you need to use either a bright point source or a clear bulb with
a long vertical linear filament (sometimes called a showcase lamp). Arrange the
positions of the source and object as suggested in the following top-view sketch.
screen
object
source
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Remove the object and then turn on the bulb, showing that the screen is (pretty
much) fully illuminated. Then turn off the bulb and replace the object as shown.
3.
?
4.
??
WHAT IS YOUR IDEA? Your instructor has a setup. Where do you think the
shadow will appear? Draw a diagram that represents your idea about how and
where a shadow is formed.
WHAT ARE THE GROUP'S IDEAS? What predictions have other members of
your group made? Sketch diagrams that differ from yours. If the group can
reach consensus, describe it here.
Most students have an intuitive feeling for where the shadow will appear on the
screen, but they may have difficulty providing explanations in terms of the
specific behavior of light, and they may have difficulty drawing appropriate
diagrams. Students will rarely invent the idea that the best type of ray diagram
in this case would be drawn from a top view or side view perspective. Most of
their diagrams will include lines, but the lines may have different meanings for
the students. In some drawings lines with arrows will be shown going from the
source to the object, implying that the object blocks light. In some diagrams lines
will be drawn emanating from the source to represent the idea that the light fills
the space surrounding the source. In other diagrams lines drawn between the
middle part of the object and screen represent the formation of the shadow (as if
the shadow went from the card to the screen). The major difficulty here is that
students have no procedure for constructing a diagram in which they can trace
light from the source past the periphery of the object and to the screen. Their
reasoning about simple shadow situations seemed dominated by past
experiences, rather than by a conceptualization of the shadow formation process
in terms of the specific behavior of light. At this point, it would be best to avoid
too much additional discussion about the properties of shadows and their
explanations. Students should be discussing these ideas as they work through
the remaining parts of the activity.
5.
MAKING OBSERVATIONS: What did you observe when the instructor
turned on the light? Was your prediction confirmed?
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6.
➥
MAKING SENSE: We would now like for you to explore some properties of
shadows. Cut out an interesting shape from a piece of cardboard. You (I) will
use this to make a shadow. (Your group can decide on one shape.) Sketch the
shape below.
Large-Class Note: You can cut out the shape yourself and show it to the class.
Make a shadow on the screen that is somewhat larger but the same shape as
the cardboard cutout. Why does the shadow region on the screen have the
same shape as the cutout? Write your own idea and then compare it with those
of your group.
When you review this question at the conclusion of this activity, it should be
brought up (hopefully by the students) that the similarity between the shadow
shape and the cross-sectional shape of the object provides strong evidence that
light travels in straight lines. This leads to the next powerful idea in the unit.
Furthermore, the observation that the shadow is larger than the object can be
explained in terms of the Light Emission from a Source powerful idea.
7.
?
WHAT IS YOUR IDEA? Consider the following questions. Answer each by
first writing down your idea. If you think you could do what is asked,
describe how you would do it. If you don't think it could be done, explain
why not.
Can you make the shadow region on the screen larger than the present shadow
you have made?
Can you make it the same size?
Can you make it much smaller?
8.
??
WHAT ARE THE GROUP'S IDEAS? Let each group member discuss his or
her ideas. List the ones below that are different from yours.
To make the shadow the same size would require essentially parallel light or
holding the object against the screen. With a single source this would mean
that you would have to move the source very far away from the object and
screen. You might suggest at this point that the students make a sketch of this
latter arrangement to see why the source must be very far from the object and
screen. If there is time, you might use this diagram to illustrate that light
coming from a source very far away is essentially parallel. However, do not
go much beyond this, because the parallel light concept is used primarily in
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defining the focal point of converging and diverging systems, concepts which are
not developed in this unit.
The students should not be able to come up with a strategy for making the
shadow smaller. Some might suggest a spread out light source. The umbra
region would be smaller in that case. However, remind them that we are looking
only at a "point" source at this time.
9.
MAKING OBSERVATIONS: Try the various ideas suggested by the members
of the group. Summarize your findings here.
10.
MAKING SENSE: In trying to understand the behavior of shadows, does it
make more sense to think of light traveling in straight lines or in curvy (or
wavy) lines. Why do you think so?
➥
The general consensus will likely lean toward the straight-line idea because it
seems to work well in predicting and accounting for locations of shadows.
However, some students may hold to an earlier wave theory of light and say that
the waves are really small; so a wavy line might be okay. You might suggest that
if we draw the waves so small that they can't be discerned, then we have
effectively drawn straight lines.
In the preceding activity an idea was suggested, namely, that light travels
outward in all directions from each point on the source. Does this idea seem to
be useful in helping you make sense of the observations in this activity?
Most students will find this useful.
Look over the diagrams you and your classmates have drawn in this activity to
represent how light behaves when a shadow is formed. Some of these
diagrams might be drawn from a "top-view" perspective, which means it is
drawn from the point of view of looking down from above on the bulb,
shadow, and screen. This diagram would show how the shadow region would
look on the table top and also the boundary region of the shadow on the
screen. Other diagrams might show a "front-view" perspective of the bulb,
shadow, and screen. Here you would see a sketch of the front view of the
whole bulb, the object, and what the shadow looks like on the screen. What
advantages do you see in drawing each of these types of diagrams? If your
purpose is to represent how light behaves in forming the shadow, do you
think that one type of diagram has an advantage over the other?
This is a good time to review the top-view diagram as probably the most useful
way to represent the shadow formations.
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©2001 American Association of Physics Teachers
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