Trash for Teaching
Science Kit #2
Pathways of Light
trashforteaching.org
OVERVIEW
This unit teaches some basics of optics. The main takeaway is that light always travels in a straight
line and will not (generally) change direction until it encounters a different material.
Students will study properties of mirrors and lenses. They will construct a pinhole camera and a
mirror that shows them the way others see them (rather than with left and right reversed, as in most
mirrors).
OBJECTIVES
Students will learn:
• That in general, light always travels in a straight line
• That in general, light only changes direction upon encountering a different material
• Some principles of mirrors, lenses, Fresnel lenses, and pinhole cameras
NOTES
Caution: Some of these experiments involve the use of bright light sources. Students should never use
the sun as a light source for any of these investigations.
In choosing a light source, try to find something that is much brighter than its surroundings and also
covers a large portion of the visual field. A brightly lit window in a darkened classroom makes an
excellent light source. If possible, place some oddly shaped objects in front of the window, to help
make it easy to distinguish images that are reversed from those that are not. An overhead light in the
classroom, or a television in a very dark room, is a good second choice. A small object like a flashlight
is usually not a good choice: since it only covers a small portion of the visual field, the large holes of
the pinhole camera cannot resolve it into an image. This is another reason why the sun is a poor
choice.
Students with visual difficulties (including difficulty distinguishing colors) can still understand most
of these concepts intuitively.
Terminology: beam (of light); concave; convex; distort; farsighted; flipped; flopped; focus; Fresnel (say
“fruh-NELL”) lens; hyperopia; image; interface; inverted; lens; mirror; myopia; nearsighted; ray; real
image; reflect; refract; reversed; thought experiment; upright; virtual image.
MATERIALS
Kit materials:
metalized plastic film sheets
rectangular plastic chip
specimen cup (with screw-on cap)
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Petri dish (clear plastic) and cover
box
pen clip
paper
Teacher kit materials:
eyeglass lens
piece of Fresnel lens
Other supplies that might be useful:
scissors
pencil, pen, or marker
glue or tape
light and dark paper
water
THE STORY
Our world would be very difficult to live in if we could not see, yet we often take the behavior of light
for granted. Nevertheless, students often understand more about the properties of light than they
think they do.
1. Invite the class to work up a definition of the word “light.” They may find that like “art,” the term
can be easy to recognize but very hard to define. Even scientists don’t know exactly what light is, nor
why it always travels at a particular speed.
2. Do the same with the word “color.” Point out some colorful objects and mention that although we
all agree that all red objects are red, for all anyone knows, the color that person A sees when looking
at a red object might be the same as the color that person B sees when looking at a blue object. Is
there a way this could be tested?
3. Explain that for the most part, light passing through any material (or through empty space) travels
in a straight line, which we call a ray of light. It only changes direction when it comes to another
material. Some materials will change the light’s direction slightly (example: reading glasses); others
will change it greatly (example: a mirror). So if we want to redirect a ray of light, we must insert a
different material in its path. (Light is also affected by gravity, but the effect is very subtle and not
noticeable in the classroom.)
4. Invite discussion on what a mirror is and how it works. Emphasize that when light strikes any part
of a mirror, it is always reflected at exactly the same angle at which it came in. Do you appear to
yourself in a mirror the same way you appear to others? Is every flat object a mirror? Is every mirror
flat? Are all mirrors the same color?
5. Draw a straight line representing a flat mirror. Draw lines approaching the mirror, representing
light rays. Ask for guidance on drawing the direction in which each ray will be reflected.
6. Invite discussion on what a lens is and how it works. How is a lens like (and unlike) a mirror?
7. The asymmetry of the letter R makes it a useful tool for demonstrating the sometimes
amusing/often confusing terminology of optics. The following terms can be used to describe an
image. Introduce each one, and draw an R in the appropriate position for each:
flipped
flopped
inverted
reflected top-to-bottom (the tail of the R points “northeast”)
reflected left-to-right (the tail of the R points “southwest”)
standing on its head but not reflected (the tail of the R points “northwest”)
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reversed
upright
reflected in any direction (for example, flipped or flopped)
in normal position (the tail of the R points “southeast”)
ACTIVITIES
Project 1: Experiment with reflection
One way to make light do useful work for us is by changing its direction. One way to change its
direction is by causing it to “bounce” it away. This is called reflecting the light, and it is what mirrors
do.
1. Look at the plastic chip. Some of its surfaces are reflective, but others are not. How are the
reflective surfaces different from the other surfaces? (Answer: They are smooth.) When you look into
the reflective surface in bright light, you might expect that everything you see will be the same color
as the chip. Is it? What color are the mirrors on a car? Does everything in those mirrors appear as a
single color?
2. Besides the chip, which of the kit materials could be used as a mirror? (Answer: Almost all of the
materials are at least partially reflective, even the flat side of the Fresnel lens. However, not every
reflective material can form an image. For instance, one piece of the metalized plastic film is silvercolored and shiny like a mirror, and yet it is difficult to see an image in it.)
3. Examine the specimen cup. Which of its surfaces are mirrors? Now pour some water into the cup.
Is the water’s surface a mirror? Does it reflect colors, or only light and dark areas? Look into the cup
from the side. Is the bottom of the cup a better mirror now than it was when the cup was empty? Now
hold the cup slightly above your eyes and look up at the underside of the water’s surface. Is it a
mirror? Where did all these mirrors come from? (Answer: Wherever the water meets another material,
such as air or the bottom of the cup, it reflects light, making a mirror.)
4. Place the Petri dish (or its cover) on a table or desk, with the rim facing down. Does it make a
mirror? Put a piece of plain paper under. Does this make it a better mirror? Which works better:
lighter or darker paper? (Answer: Darker, because lighter paper contributes more additional light to
the image.) Have a classmate sit opposite you. Each of you should hold a pencil and use the other
hand to cover one eye (it doesn’t matter which hand or which eye). Move your heads until each of you
can see the other’s eye reflected in the Petri dish. Holding very still, use your pencils to touch the
point on the mirror where you see the pupil of the other person’s eye. Change positions slightly, or
move the mirror slightly, and do this again. Do you both touch the same point every time? Why is
this? (Answer: The path the light takes to go from your eye to your classmate’s is the same as the
path it takes to go from your classmate’s to yours. It strikes the mirror at the same point in both
directions.)
5. Now while your classmate holds still, move slowly in any direction. At the moment you can no
longer see the reflection of your classmate’s eye, freeze. What did your classmate just see? Why?
(Answer: From his or her point of view, the image of your eye has just moved off the edge of the
mirror.)
Project 2: Experiment with mirrors that are not flat
1. Look at the pen clip. Part of it is fairly flat. When you look at it, you can see the reflection of your
eyes. Part of it is curved. What do you see when you look into the curved part? What happens when
you turn the clip sideways?
2. Take the metalized plastic film that has the larger (1") squares. Look at your own reflection in both
sides. Are there differences? On each side, do you appear upright or inverted? Do you appear
reversed? (This is easier to determine if you’re wearing something with words on it, or if you hold up
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one hand.) Do you appear the same size on both sides? Do these effects change when you rotate the
sheet? (Answer: You appear flopped and smaller on one side. You appear flipped and larger on the
other. Rotating the sheet has no effect.)
3. The film works the way it does because it is not perfectly flat. On one side, all the squares bulge
outward a little bit, like a pillow. On the other side, all the squares curve inward a little bit, like the
inside of a bowl. The ridges in between are the only places where the sheet is flat, and they are a little
thicker. You may be able to feel their thickness with your fingers, or by rolling the film into a tube,
being careful to keep the squares aligned.
4. The outward-curving side is the one where you appear flopped and smaller. Another word for
“outward-curving” is convex; each square on this side is a convex mirror. The inward-curving side is
the one where you appear flipped and larger. Another word for “inward-curving” is concave; each
square on this side is a concave mirror.
5. Place the Petri dish (or its cover) on a piece of dark paper on a desk or table, with the rim facing
up. Now look down into it and move your finger around the rim. Does the reflection of your finger
appear near the center of the dish, or close to its edge? (Answer: Close to the edge.)
6. Find one unwrinkled square in the metalized plastic film and cut it out carefully. Examine the two
faces of the square. One face is a convex mirror; the other face is a concave mirror. Place the square
in the center of the Petri dish with the convex side facing up. Again, look down into it and move your
finger around the rim. Where does your finger’s reflection appear now? (Answer: Much closer to the
center of the dish.)
7. Turn the film square over so the concave side is facing up. Move your finger around the rim of the
dish as before. Can you see the reflection of your finger at all? (Answer: It is far beyond the edge of
the dish.)
8. Use these diagrams to help explain the behavior of each of the mirrors. The thick lines are mirrors;
the thin lines are light rays:
9. Students sometimes think that the reason a concave mirror reflects things upside-down is that the
mirror is reflecting itself; that is, that light strikes the mirror, bounces to a different part of the
mirror, then bounces to the eye. Although this may be the case in a highly concave object such as a
silver bowl, it is not generally true of shallow concave mirrors such as the one we are using. Use the
diagrams above to explain why. How could you tell if the mirror were reflecting itself? (Answer: The
right-hand diagram above shows that light only strikes the mirror once. If the mirror were reflecting
itself, then light would strike it twice, and the image would be inverted, not flipped.)
10. Repeat some or all of the above with the metalized plastic film that has the smaller squares. Is the
effect the same? (Answer: Yes, but the size differences are greater.)
11. Repeat the two-person experiment from Project 1, using a concave or convex mirror.
Project 3: Experiment with a distorted mirror
1. Cut a square out of the metalized plastic film sheet that has larger squares, and include part of the
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squares around it. Hold it in both hands, with either side facing you. Experiment with bending it
inward and outward along various directions (this is called distorting the mirror). What happens? Try
to make yourself appear in each of the following configurations, and determine whether each image is
flipped, flopped, inverted, or upright:
•
•
•
•
an upside-down image above a right-side up image (tops of heads nearly touching)
a right-side up image above an upside-down image (like a queen or king in a deck of cards)
two images side-by-side
one image in which some parts are stretched out, others compressed (example: a very long
neck and a flat head)
• three images of yourself
• more than three images of yourself
2. Discuss the ways in which this activity is like looking into a carnival funhouse mirror.
3. Glue or tape the square to the outside of the specimen cup, concave side out. Try to keep the film
as smooth as possible. Holding the cup sideways (Figure ??), look into the mirror and describe what
you see. Raise your left hand. Which hand does your image raise? (Answer: You see yourself upright,
not reversed as in a normal mirror. When you raise your left hand, your image raises its left hand.)
4. You have created a mirror that shows you the way you look to others. What causes this rather
surprising effect? (Answer: The curve of the specimen cup forces the mirror to be concave in one
direction but convex in the other.)
5. Turn the cup mouth-up. Now what do you see? Why? (Answer: Your image is now inverted, but it
is still not reversed. When you raise your left hand, your image raises its left hand. You see yourself
the way others would see you if you were standing on your head. This happens because the concave
and convex directions change places.)
6. Can you think of another way in which the film could be distorted to produce these effects?
(Answer: Look into the convex side and curve it to make it concave along one axis. This may be easier
to do with one of the smaller squares; hold it by the edges between thumb and forefinger, with the
convex side facing you, and squeeze until it bends away from you into almost a semicircle.)
Project 4: Experiment with refraction
Another way to change the direction of light is by “bending” it. This is called refraction. It is important
for students to understand that refraction is generally a sharp, sudden change of direction (not a
gradual curving) and that it happens at the interface (or boundary) between two different materials. A
single material alone does not refract light (unless it has a varying density). The thickness of the
materials also does not matter: within a given material, light travels in a straight line. It refracted
only when it leaves the material.
This may be surprising, because Project 1 showed that the interfaces between water and air, and
between water and the cup bottom, made a mirror. The explanation is that the interface between two
materials can both reflect and refract light, depending on the light’s angle and direction of travel.
A lens is a transparent object whose shape is carefully designed so that light entering and exiting it
will be refracted in a precise way. Most of the cameras students are familiar with (including camera
phones) have at least one lens. The human eye also has a lens. In both cameras and eyes, the lens
focuses light to make an image.
1. Which of the kit materials can bend light? (Answer: The specimen cup, the Fresnel lens, and the
eyeglass lens are obvious answers, but anything that is transparent or translucent can bend light.)
2. Some of the specimen cups have markings on the side. If yours has markings, find a line marked
“40” and use a pen (or a speck of tape) to make a large dot there. On the opposite side of the cup, use
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a pen (or two slivers of tape) to make an X at the line labeled “80.” Try to work around the label, if
any, or else peel it off. If your cup does not have markings, estimate where the center of the cup is
and make a dot ½" below it on one side and an X ½" above it on the other.
3. Place the specimen cup on a desk or table, with the dot facing away from you and the X facing
toward you. Move your head or the cup until you can see the dot when you look through the X.
4. Without moving your head, fill the specimen cup with water to the level of the X. Where is the dot
now? (It is no longer visible behind the X; instead, you can see at least two images of the dot: one
through the surface of the water, the other through the side of the cup.) Why are you able to see
multiple images of the dot? (Answer: Light travels in a straight line until it comes to the interface
between materials, then it bends.)
5. Figure ?NUMBER? shows light starting from the dot, then
following two different paths (the solid arrows), each of which
bends, and ending up at the viewer’s eye. The dashed arrow
shows the path of the light from the dot when the cup was
empty. Use the diagram to explain why one image of the dot
appears to be higher up than it did before, while the other
one appears to be lower down. (Answer: One of the solid
arrows ending at the eye comes in at a higher angle than the
dashed arrow, so we have to look up a bit to see that image
of the dot . The other solid arrow ending at the eye comes in
at a lower angle than the dashed arrow, so we have to look
down a bit to see that image of the dot.)
6. Why doesn’t the dot seem to be behind the X anymore?
What happens to a ray of light that starts out along the
dashed arrow? (Answer: The ray is refracted when it leaves
the water, and it no longer ends at the eye.)
7. You might have seen a third image of the dot down below
the other two. What causes this? Draw the path of the light.
(Answer: Light traveling downward from the dot is reflected
off the interface between the water and the bottom of the
cup, bounces upward, and is refracted at the edge of the cup
as before. You would need three arrows to draw its entire
journey; see Figure ?NUMBER?.)
8. If another person were to put his eye at the position of the
dot and look up at you through the cup, would he see two
images of your eye? And if he looked down, would he see a
third one? (Answer: Yes; light traveling from your eye to the
dot follows the exact same paths as light traveling from the
dot to your eye—just in the opposite direction. The light is
reflected and refracted through the same angles no matter
which direction it is traveling in.)
9. The light actually passes through three materials in this
experiment: water, air, and the plastic of the cup. There are also three interfaces: water/plastic,
water/air, and plastic/air. Which of the three interfaces refracts light the least? The most? (Answer:
Plastic/air refracts light the least, as we saw when we looked through the walls of the empty cup.
Water/plastic refracts light the most. We know this because the lower image of the dot, which passes
through the water/plastic interface, is more distorted than the upper image, which passes through
the water/air interface.)
Project 5: Experiment with lenses
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1. Some people require vision correction, in the form of glasses, contact lenses, lens implants, etc.
Discuss the difference between nearsightedness (myopia) and farsightedness (hyperopia): nearsighted
people have difficulty focusing on things that are not near their eyes; farsighted people have difficulty
focusing on things that are not far from their eyes. If you meet a person who is wearing glasses, you
can’t always tell at a glance whether the person is nearsighted or farsighted; but the glasses
themselves will tell you. One way to know is by seeing what the glasses do with light. Glasses for
nearsighted people spread light out; glasses for farsighted people shrink it down.
2. If you have eyeglasses (not contact lenses), take them off and inspect them. (Or you can use the
eyeglass lenses that come with the classroom kit.) Hold them at arm’s length and look through them.
Do they distort images? In what way? Do both lenses appear to distort images equally? (Some
people’s eyes require different corrections.)
3. When you look through a lens, what you see is called a “virtual image.” Some lenses, such as the
ones in cameras and movie projectors, can also project an image onto a surface, so that you can see
it without looking into the lens. Such an image is called a “real image.” Magnifying glasses, such as
reading glasses or glasses for farsighted people, can produce a real image. Glasses for nearsighted
people cannot. Both kinds of glasses can produce a virtual image.
4. Hold the lens (or one lens of the glasses) horizontally over a desk or table under a bright light (not
the sun) and look at the shadow that is cast by the edges of the lens. Do you see a bright area inside
the shadow? What happens as you move the lens closer to or farther away from the table? Can you
make an image of the overhead light appear on the table? (Not always!) What happens if you flip the
lens over? (Answer: The effect is the same.)
5. Collect the students’ responses and discuss the different results. What is the connection between
the bright area inside the shadow, the type of lens, and the person’s vision? (Answer: If the bright
area is smaller than the shadow of the lens (Figure ??), the glasses can make a real image, and the
person is farsighted. If the bright area is larger than the shadow of the lens (Figure ??), then the
glasses cannot make a real image, and the person is nearsighted.) From the behavior of the lenses,
can the class determine which (if any) of the lenses in the teacher’s kit are for nearsightedness, which
are for farsightedness, and which make no correction at all?
6. Why might a person wear glasses that correct neither nearsightedness nor farsightedness?
(Possible answers: Sunglasses, safety glasses, fashion glasses, polarized lenses for viewing 3D
movies.)
7. How might you determine whether a person is nearsighted or farsighted without touching his
glasses? (Possible answers: If things seen through the person’s glasses appear larger than they really
are, such as his eyes or ears or anything behind his head, he is farsighted. — If the lenses are
obviously thicker in the middle than they are toward the edges, he is farsighted. — If he puts his
glasses on only for reading, he is probably farsighted. — If he is elderly, he is more likely to be
farsighted. — If he doesn’t have his glasses on and he holds things at arm’s length to read them, he is
probably farsighted. — If he doesn’t have his glasses on and he holds things close to read them or
squints at things that are far away, he is probably nearsighted. — You could always ask him!)
8. A certain person needs reading glasses and yet is nearsighted. How is this possible? (Answer: He
wears contact lenses to correct his extreme nearsightedness, but this changes his range of vision so
that things close to his eyes are no longer in focus. When he needs to read, rather than taking his
contacts out, he puts on his reading glasses. This is the author of this science kit!)
9. You can form a real image with a magnifying lens, such as with the lenses from reading glasses.
But suppose you have only a piece of a lens. Will it still make an entire image, or will it only make
part of an image? (Answer: You do not need an entire lens in order to form an image. Each part of the
lens directs light all over the image; thus, if you have only part of a lens, you will see a complete
image, not part of an image, but it will be dimmer.)
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10. The Fresnel lens is a specially formed magnifying lens, and therefore it too can make a real
image. The teacher’s kit comes with one or more chips cut from a large Fresnel lens that originally
measured 8 inches by 10 inches. Inspect a chip and describe its surfaces. Hold the chip horizontally
over a desk or table with the smooth side down and move it up or down until it makes an image of an
overhead light. (The image may be easier to see if you can direct it into a shadowy area, such as the
interior of the long box from the kit.) When you did this with glasses for farsightedness, the image
was directly under the center of the lens, but with the Fresnel lens, the image may be off to one side.
Why? What happens when you rotate the lens? (Answer: A lens forms a single image, and each part
of the lens contributes to that image, If you had the complete Fresnel lens, the image would form
under its center, but since you only have a piece of a lens, the image forms at the place where the
center of the complete lens would be.)
11. Stand directly under the light and measure (or estimate) the horizontal distance between the
center of the image and the center of your chip (Figure ??). What does this distance represent?
(Answer: If you were holding the original lens, the image would have formed directly under its center,
so this distance tells you how far your chip was from the center of the lens.)
12. If the class has more than one Fresnel chip, compare the results from each and try to reconstruct
where each was in relation to the center of the original lens. Now inspect the curves in the chips.
What do you notice? (Answer: The closer a chip was to the center of the original lens, the tighter the
curves in its grooves, and the closer the image is to the lens’s center.) Suppose your chip contains a
groove that makes a complete circle. What is at the center of that circle? (Answer: The center of the
original lens.)
Project 6: Study how a pinhole camera works
1. Mirrors and lenses bend light. But we can make light do useful work without bending it. This is
how pinhole cameras work. A pinhole camera has no lens; instead, a small hole masks off most of the
light coming in, allowing only a little bit of the light to pass through. (Despite the name, the “pinhole”
in a pinhole camera can be larger than the tip of a pin.) The small amount that passes through forms
an image—not of the hole, but of the source of the light. To understand this better, we will do what
scientists call a “thought experiment”: an experiment you do entirely in your head. The only
equipment you will need is your imagination.
2. Start by imagining a huge crowd of ants all gathered on a table under a bright lamp shaped like a
square. Every ant, no matter where he is, sees the entire lamp when he looks up (Figure ?LEFT?).
3. Now imagine that you have a piece of cardboard with a hole in it and are holding it just above the
table (Figure ?CENTER?). Most of the ants are now in shadow; if they look up through the hole in the
piece of cardboard, they won’t see the lamp. There are only a few ants who can see the lamp, or at
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least a part of it, through the hole. Those ants are all inside a bright area on the ground (the
“shadow”of the hole).
4. Next, imagine slowly raising the cardboard away from the table (Figure ?RIGHT?). From the ants’
point of view, the hole is masking off more of the lamp than before. An ant looking up through the
hole either won’t see the lamp at all or will see at most a small part of it, depending on where he is.
5. At some point, we will see an image of the lamp on the table. Each ant within the image is being lit
up by only one small part of the lamp. Is the image inverted? Upright? Flipped? Flopped? To decide,
imagine you have labeled the four edges of the table NORTH, EAST, SOUTH, and WEST, and that you
are standing at the SOUTH edge, looking at the table as if it were a map. Now imagine tilting your
head back to look up at the lamp. From this point of view, what you
see as the lower right corner of the lamp is actually its northeast
corner (it is over the northeast corner of the table). If you were to
paint a dot on that corner of the lamp, only the ants near the
southwest corner of the table (the corner closest to your left hand)
could see it through the hole and would be in its shadow. In other
words, the lower right corner of the lamp appears in the lower left
corner of the image. If you painted an R on the lamp (with the tail
pointing toward the dot you made in the lamp’s northeast corner),
the image on the table would look like this: Я (with the tail pointing
toward the southwest). (This result is not always obvious, and
visualizing it is often the hardest part of the thought experiment. It
may help to have students do the following: On the top half of a
piece of paper, draw a large R . Below it, draw a large backwards R
(that is, Я ). Fold the paper in half horizontally, then unfold it
slightly and peek in. Looking up, you can see the R on the lamp;
looking down, you can see the Я of the image; and between them,
imagine the piece of cardboard with its hole (Figure ?NUMBER?).
Notice that every part of the Я lies across the hole from the
corresponding part of the R .)
6. With your imaginary lamp, cardboard, and table, you have made a pinhole camera. Unlike other
cameras it will not be used for taking photos (although it could be), but like them it can make an
image of a bright object. Is the bright area in the center diagram an image? (Answer: No, it is the
“shadow” of the hole.) If you had used a circular lamp and a square hole, what shape would the
bright areas in Figure ?CENTER? and Figure ?RIGHT? be? (Answer: A square and a circle,
respectively.)
7. What would happen to the image in Figure ?RIGHT? if you made the hole larger? Use the ants to
help you explain. (Answer: It would get brighter, because an ant looking through the hole would see a
larger area of the lamp. But it would get fuzzier, because ants nearby would also see part of that
same area of the lamp. Now the dot of paint on the lamp would cast its shadow on several ants
instead of just one.) What would happen if you made the hole smaller? (Answer: The image would get
sharper but dimmer.) So what is the best size for the hole? (Answer: The “best” size is whatever
makes the image not too dim but not too fuzzy. Only you, the viewer, can decide what you think is
“not too dim but not too fuzzy.”)
Project 7: Build and test a pinhole camera
1. Look at the cardboard box. Observe that there are two holes near one end. Can you guess what
their original purpose was? (Answer: They held a decorative ribbon.)
2. Open the box at the end nearer the holes (the top). Notice that the holes also go through the large
flap. Cut off part of that flap (just the part that has the holes in it), leaving enough of the flap so that
the box can be reclosed. Save the piece you cut off. Close the box and make sure that no part of the
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flap is blocking the holes.
3. Select a bright light source, such as a bright window in a darkened room.
CAUTION: Avoid looking directly through the holes when using the pinhole camera with any bright light
source. Never use the sun as your light source.
4. Open the box at the end farther from the holes (the bottom) and bend the flaps back out of the
way. If your light source is not overhead, such as a standing lamp or a brightly lit window, then point
the closed end of the box toward the floor. Look into the box and turn it so that the two holes are
pointing toward the light source. You should see two bright spots of light inside the box. You will
probably decide that they are the “shadows” of the holes.
5. Cover one of the two holes with your finger or a piece of paper. One of the bright spots should
disappear. Without lifting your finger, slowly move and turn the box. As you move, the spot will move.
What happens to the spot as it gets closer to your end of the box (the end near your face)? Why?
(Answer: It looks less like the “shadow” of a hole and more like an image of the light source. The
closer the image gets to your end, the larger, fainter, and sharper it becomes. This is similar to what
happened in our thought experiment when you raised the cardboard above the table.)
6. Continue moving and turning the box, keeping one hole covered, until the bright spot is very close
to your end of the box. If the light source you chose is a window or a light fixture with a distinctive
shape, the image will probably be recognizable right away. If it is something relatively featureless or
symmetrical, then have a friend hold up a hand in front of the light source and wiggle his fingers.
Can you see the hand in the image?
7. What do you notice about the image? Is it larger or smaller than the actual light source? Is it
upright or inverted? Is it reversed? What happens if you point the closed end of the box in a different
direction while still keeping the holes pointed toward the light source? (Answer: The image is reversed
and is usually smaller than the actual light source. Pointing the box in different directions causes the
image to rotate around its center.)
Project 8: Modify the pinhole camera
1. Make the hole a little smaller by covering part of it with your finger. What do you notice? (Answer:
The image gets fainter but sharper.)
2. Move your fingers out of the way so that both holes are completely uncovered. What happens?
(Answer: You see two images.)
3. Take the piece of the flap you cut off earlier. Cut it into three small squares, each about half an
inch on a side. Tape one of the squares to the box so that it covers half of one of the holes; this turns
the hole into a smaller hole. Place a second square so that it completely covers that hole. Tape it on
one side only, to make a hinged flap that you can open and close. Finally, place the third square so
that it completely covers the other hole, and tape it on one side only to make a hinge.
4. Use the hinged flaps to experiment with the holes. Cover the smaller hole and uncover the larger
one. Cover the larger hole and uncover the smaller one. Uncover both holes. One of the holes
produces a fainter but sharper image. The other produces a brighter but fuzzier image. Which is
which, and why? (Answer: The smaller hole produces a fainter but sharper image. As with the ants,
when the hole is smaller, each place on the wall of the box “sees” a smaller part of the light source.)
5. Use a pencil or similar object to poke a small hole through the center of the closed end of the box.
Halfway down the box, cut a slit through any two adjacent sides. Slip a piece of paper into the slit.
Point the end of the box at your light source and look into the box. Can you see an image on the piece
of paper? Is it reversed? (Answer: No, because you are viewing it “from the other side.”)
© 2011 Trash for Teaching
Pathways of Light
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6. Move closer to your light source. What happens to the image? (Answer: It becomes larger.) Explain
this in terms of our thought experiment with a table full of ants.
7. Take the box away from your face, remove the piece of paper from the slot, hold the paper so that
it covers the open end of the box, and without putting the box to your face, point the closed end of
the box at the light source again. What happens to the image? (Answer: It is larger, fainter, and
sharper.) Explain this in terms of the thought experiment.
Project 9: Light expo
Conduct a light expo: a form of show-and-tell in which students present information and
demonstrations about the behavior of light, perhaps using mirrors, lenses, or similar objects. Take
advantage of opportunities to reinforce the lessons of this kit, especially the concept that light travels
in a straight line unless reflected (bounced) or refracted (bent).
Some ideas:
1. Make a pinhole camera that permits the user to make a permanent copy of the image by tracing it
on a piece of paper (for example, a shoe box with a hole in the lid and one end removed). Make some
tracings using your camera.
2. Explain how a kaleidoscope or a “hall of mirrors” works. Diagram the paths of the light rays.
3. Set up two small mirrors facing each other, such as is often found in beauty salons. Diagram the
path of the light bouncing between them. Why can’t you see infinitely far when you look straight into
one of them? (Answers may include: Your head gets in the way; the glass of the mirrors makes them
slightly dark; there is dust in the air; light gets dimmer with distance.)
4. Use two small mirrors to construct a periscope. Demonstrate it and diagram how it works. If
someone in a submarine wants to look in a different direction, he must turn the entire periscope.
Demonstrate and diagram what would happen if only the top half of the periscope turned. (Answer:
The image would become inverted.)
5. Arrange a pair of mirrors perpendicular to each other so that, when looked into, they show the
viewer as he or she appears to others (i.e., not flopped). No matter where you move, where does your
face always appear? (Answer: At the junction of the two mirrors.) Diagram why this is.
6. Arrange three mirrors perpendicular to one another to make a “retroreflector.” One property of this
device is that light traveling into it at any angle always returns via the same path. No matter how you
turn the retroreflector, what always appears at the junction of the three mirrors? (Answer: Your eye.)
7. A parabolic reflector is a reflector shaped in such a way that parallel light rays coming straight into
it are all directed to a single point (the focus). Diagram this property and explain why it can be useful
in both directions. Describe some common uses of parabolic reflectors. (Examples: flashlights;
headlights; telescopes; solar furnaces; radio antennas; satellite dishes.)
8. Explain the difference between a reflecting telescope and a refracting telescope. Describe how they
work. Diagram the path of the light inside them.
9. Draw some interesting or unusual shapes and diagram how light would bounce off them if their
surfaces were reflective.
10. You can make a sun-safe pinhole camera as follows: Poke a small hole in a large piece of
cardboard. Place a piece of paper in the sunshine and, keeping your back to the sun, hold the
cardboard up so that its shadow falls on the paper. Move the cardboard until a clear image of the sun
appears on the paper. Find out when there will be a solar eclipse (full or partial) in your area, use this
setup to view it, and describe what you saw. For viewing the sun, why does the hole need to be so
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Pathways of Light
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tiny? (Answer: The reason is not safety. It is because the sun covers such a small portion of the sky.
Recall our thought experiment: from every ant’s point of view, the hole must appear much smaller
than the light source.)
FOR FURTHER THOUGHT
1. How was each item in the kit constructed? What might its original purpose have been? What
alternate uses do you see for it? (See “Materials” sheet.)
2. Discuss any of the following:
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Where might you see the warning OBJECTS IN MIRROR ARE CLOSER THAN THEY
APPEAR, and why?
Where might you see the word AMBULANCE written backwards, and why?
Does a pinhole camera only make an image of a bright light source, or does it make an
image of everything on the other side of the hole? Why can you often see only the brightest
parts?
The distance from the hole of your pinhole camera to the image is only a few inches.
Usually, the object you are viewing is farther away from the hole than this distance, so the
image is smaller than the actual object. How could you make an image that is larger than
the actual object? (Answer: Bring the object closer to the hole than the distance from the
hole to the image. This works best when the hole is very small.)
What is the gray area you see in the four corners of the convex side of a square of the
metalized plastic film?
3. Discuss any of the following fairly philosophical topics:
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How does light “know” to bounce off a mirror at the same angle as it came in at?
Why does light “want” to bend at the boundary between materials?
Why do we notice a flicker of light in a darkened room but not a flicker of darkness in a
bright room? For instance, why do we see the frames of a movie but not the moments of
darkness between them?
How would you explain color to a person who had never been able to see?
Lenses and mirrors change the direction of light, but a pinhole camera doesn’t; it just
subtracts light. Does that mean that the image it makes also exists—in exactly the same
place—even when the pinhole isn’t there? Why can’t we see it without the pinhole? Are we
completely surrounded by zillions of tiny images of every light source around us?
RESEARCH PROJECTS
1. Light has several fascinating properties. Research some of these, such as:
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Refraction is responsible for rainbows and for the fact that a prism can split white light
into colors.
Light always travels at the same apparent speed, even if the viewer is moving. (This was
one of the key discoveries that led Einstein to his Theory of Relativity.)
Light is “bent” by gravity.
Light can be turned into heat.
Light exerts a tiny force (called “light pressure”) which could be used to move spaceships.
2. Research the construction of mirrors:
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How are they made? What materials are used? How are they made scratch-resistant?
How were they made in ancient times?
What’s a “first-surface” mirror? What are some of its advantages and disadvantages?
What are some of the largest mirrors now in use? The smallest? How are they
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Pathways of Light
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•
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manufactured and operated?
What are some common uses of convex mirrors? Concave mirrors?
Explain how a spinning dish of liquid mercury can be used as a telescope mirror.
The image that you see in a mirror is, like the image that you see in a lens, called a “virtual
image.” Some mirrors can also create a “real image” (one that can be projected onto a
surface). Explain how this works.
3. Research the construction of lenses:
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How are they made? What materials are used?
How were they made in ancient times?
What are bifocals and trifocals? What are they used for?
What are some of the largest lenses now in use? The smallest? How are they manufactured
and operated?
There is a lens inside the human eye. What do the following terms mean, what causes
them, and how can they be treated?: myopia, hyperopia, presbyopia, astigmatism, cataract.
Explain the principle behind the Fresnel lens. Describe some uses.
4. Imagine a transparent cylinder with a light source inside and mirror-polished ends. The light
bounces back and forth between the ends, building up in intensity. One end is not a perfect mirror
and lets some of the light through. How is this similar to the way a laser works?
5. How are retroreflectors used in highway signs and on the surface of the Moon?
6. Research the history of cameras. You may have to dig fairly deep: the principle behind the pinhole
camera was first noticed thousands of years ago! Explain what a camera obscura is.
7. What are the different types, causes, and effects of color-blindness? Why is it sometimes
considered advantageous (such as in certain military applications)?
8. What are mirages? How does the interaction of air and light cause them?
9. When you are underwater in a pool and you look up toward the surface, you can sometimes see
three things, depending on where you look: the sky, the reflection of the bottom of the pool, and a
thin dark area in between. Explain how reflection and refraction cause each of these.
10. Students with an artistic flair might enjoy making drawings, transparencies, or other
constructions to illustrate principles of light and color.
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Pathways of Light
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