Shadows & Reflections
Today’s Goal: To understand how shadows are made and how light is reflected.
Discussion:
> Give an example where we use a transparent material. Explain why this is useful.
> Give an example where we use a translucent material. Explain why this is useful.
> Standing in a forest you can hear the birds, but you frequently can’t see them. Why?
Sharp and fuzzy shadows
Why are some shadows sharp while others are fuzzy? Sometimes your shadow is very
distinct; other times it is so fuzzy you can hardly see it at all. What decides this?
Materials
 1 light station
 1 diffusing mask for front of light station
 piece plain white paper
 wooden rectangular block, with rod and plastic base
 filament viewer (frosted plastic rectangle)
 view screen with stands
What to do
Put white paper on the table in front of the light
station.
1. Observe the shadow that the wooden rod makes on
the paper. Adjust the wheel on the bottom of the light
station through its full range of motion (it turns about
180). Describe how this adjustment affects the
sharpness of the shadow edges.
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2. Using the filament viewer, look briefly at the glowing light bulb through the front
opening of the light station, turn the adjusting wheel and observe how this makes the
filament rotate.
Find the two filament positions shown below, and observe which one makes the
rod’s shadow edges sharp, and which one makes the edges fuzzy. Fill in the blanks.
Filament turned like this
makes shadow sides _______________________
Filament turned like this
makes shadow sides __________________________
Working with the view screen upright
Put the object close to the light and stand
the view screen upright. Notice how the
shadow sharpness on the screen changes
when the filament is turned to different
positions.
3. How does the shadow change when you put in the “diffusing mask” (the piece of
translucent plastic mounted in a frame)?
4. If you have two light bulbs, the same object can have two shadows at once. Find an
example, and then construct some more. Some parts of these shadows are darker than
other parts. Why does that happen?
5. Suppose you were at a fireworks display, and a rocket has
just produced 100 brilliant lights in the sky. What would your
shadow look like?
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6. Make shadows of a pencil with the tube light. Why does it matter how you hold the
pencil?
7. Give rules that determine whether shadows are sharp or fuzzy. They should account
for what you observed in Q1-Q6.
8. Here is a diagram that shows a light source, an object, and a viewing screen.
Complete the diagram to show where the shadow is.
source
object
viewing screen
The shadow is somewhat fuzzy. How does your diagram explain this?
9. Here is another light source, object, and viewing screen. Complete the diagram to
show that the shadow is less fuzzy when the object is closer to the screen.
Check #1: Discuss your rule and diagrams with an instructor.
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How big is a shadow?
Your shadow can be many different shapes, but also different sizes. What determines
how big a shadow is? Let's find out!
Materials
For each group of four students
 1 light station
 1 view screen with 2 stands
 1 wooden cylinder
 1 wooden rod and object stand
 metric grid paper
 2 clips to attach grid paper to screen
 meter stick or tape
 ruler
What to do




Set up the light station and view screen about one meter apart.
Put one of the screen supports flat on the table holding the bottom of the screen.
Clip the grid paper to the screen.
Adjust the wheel on the bottom
of the light station so the sides of
the cylinder’s shadow are sharp.
1. Explore shadow size. How does the
position of an object affect the size of its
shadow?
2. Using the wood cylinder (standing vertically, like the diagram shows), determine how
the width of a shadow depends on how close the object is to the screen. Measure (using
the metric grid paper or a ruler) the width of the cylinder’s shadow for about 6 to 8
different locations of the cylinder, covering the full range of positions between the screen
and light. Record each measurement in the table.
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3. Make a line graph of your data showing how the shadow width depends on the
position of the object.
4. Interpret your graph. That means to explain what the graph tells you about the
relationship between your dependent and independent variables.
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Sharp and fuzzy shadows (again!)
Materials
 1 light station
 1 diffusing mask for front of light station
 piece plain white paper
 wooden rectangular block, with rod and plastic base
 filament viewer (frosted plastic rectangle)
 view screen with stands
 frosted light bulb
 masks B, and C
Put the view screen on a stand, so that it is vertical.
1. Place the diffusing mask over the front of the light station. As we have already
observed, the shadow is very fuzzy. How will the shadow change if you place masks B
or C next to the diffusing mask? Predict, and then try it.
2. This diagram shows a light station with a diffusing mask, an object, and a viewing
screen. Draw some lines representing light beams to explain why the shadow is very
fuzzy.
Check #2: Discuss your finding with an instructor.
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Where does reflected light go?
When light hits a mirror, it changes direction. But which way does it go? How do you
figure out where it will go?
Materials
 light station
 single slot mask
 2 mirror holders
 white paper
 white screen (no stands)
 angle card
Avoid getting fingerprints on the mirror. Hold it by the edges.
What to do
Make a light beam
 Lay the white screen flat on the table close to the light
station.
 Slide the single slot mask over the light station opening
to make a beam across the paper.
 Turn the light station adjusting wheel through its full
range of motion, to find the best setting to make a
narrow, sharp beam.
1. Explore how a mirror affects a light beam.
card? How is it similar?
How is the mirror different from a white
2. Observe how the mirror’s orientation (how it is turned) affects a light beam. Record
two observations about a light beam reflecting from a mirror.
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3. To find a rule about where reflected light
goes, measure some angles and look for a
pattern of behavior. To do this:
 Attach holders to the mirror’s sides
(see picture)
 Stand the mirror in its place on the
angle card.
 Move the mirror and angle card into
the beam until the beam hits the
vertex of the angle grid.
Read the angle grid to measure the light’s
incident angle (A) and the reflected angle (B)
and record these in the table.
Move the mirror and angle card so that the light strikes the mirror from a different
direction, but always at the vertex.
Repeat your measurements and record them too, for 5 different mirror positions.
Angle of the incident beam (A)
(degrees)
Angle of the reflected beam
(B) (degrees)
4. Look for a pattern in your table. Use that pattern to give a rule for reflection telling
how the directions of the incident and reflected light beams are related
5. Check your rule by placing the mirror at a different angle to the light beam and
measuring the angle of the reflected beam.
Check #3: Discuss your finding with an instructor.
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6. Suppose there are two mirrors at right angles to each other, and a light beam that
comes in from some direction (as shown in the diagram). After the light beam has
reflected from both mirrors, which way is it going?
7. One of the first measurements of the size of the earth depended on a curious fact:
right at noon on a certain day of the year in a certain city in Africa, you can see the sun
reflected from the water at the bottom of deep
wells.
A) Indicate on the diagram the path that sunlight
follows to get to the viewer’s eyes.
B) What was special about this day for this city
in Africa?
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C) It turns out that conditions are never right in Lexington so that you can see the sun’s
reflection from the bottom of a deep vertical well. Suppose the well were dug at a slant:
would you be able to see the sun's reflection, perhaps at some other time or on some other
day? Please draw the path for the sunlight, and explain your answer.
Check #4: Discuss your thought with an instructor.
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Group:
Names of group members present:
1. Explain how to use your graph (in the Shadow part) to infer where you would need to
place the cylinder in order to make a shadow that is 10 cm wide.
2. Why did the instructions say to observe the width of the shadow and not the height?
3. Louise has a great idea how to decorate her living room for when she hosts the bridge
club next week: she will draw hearts and spades on the light bulbs, so that they project
these pictures onto the lampshade and the ceiling. Easy to do, cheap, and after the party
she can just wash the pattern off. Her neighbor Margie objects, “If that’s going to work,
why doesn't it say '100 Watts' on the ceiling already?” Is Margie right that this won't
work? Or can you tell Louise how to rescue the scheme?
6. This top view diagram shows a light beam coming straight out from a light station.
There is a mirror off to the side not doing anything. You can move the mirror anywhere
that you want in the picture. Draw the mirror somewhere in the light beam, showing it
turned at the correct angle so that the beam is reflected and goes through the black X.
Draw the reflected beam, too.
Light beam
Explain how you determined where to put the mirror and how to turn it.
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Check #3:
The angles A and B come out equal to each
other.
There are other ways to state the rule, if you
measure other pairs of angles. If you measure
both angles from the same side of the mirror, you
will get two numbers that add up to 180.
The important point is that a mirror changes the
direction of a light beam into a new but very
definite direction. In contrast, light reflects from
a piece of paper to go many directions at once
(which is why we can see the paper).
We use mirrors because we see a familiar-looking person when we look at one. That can
be explained using the rule you found (we will explore the connection soon).
Check #4:
In this situation, light reflects twice, and we use the
“equal angle” rule twice. If the two mirrors are exactly
at right angles, the outgoing light beam is parallel to the
incoming one.
When the astronauts went to the moon, they left behind
a reflector that is based on this idea (theirs has three
mirrors and works for light coming from any direction).
Now astronomers on earth can reflect light from the
mirror on the moon. By timing how long it takes for the
round trip, they can measure the distance to the moon,
accurate to a few centimeters.
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