All About Shadows
Topic
Light travels in straight lines
Introduction
It may seem a strange idea that light travels only in straight lines, but it would be
even odder if it were able to curve around objects in its path to illuminate them
from behind. Where would we be without our shadow – the area of darkness
behind us where our body blocks out the rays of light from the Sun? You can
estimate the time of day by judging the length of shadows; shadows decrease in
size towards midday when the Sun is high in the sky, and increase in length as
the Sun sinks to the horizon. In this experiment, you will make shadows using
electric light as a source of light. You will then find out how to make shadows
larger and smaller.
This experiment and others in this section require a narrow, concentrated beam
of light (a ray of light) whose path remains visible for at least 0.5 meter and
which does not “splay out” (or diverge) at the sides. This is achieved using a
piece of equipment known as a “light box” or a “ray box” or a “ray optics
box.” If you do not want to purchase a light box like the one shown in
diagram 1 below, you can make one using the instructions given for Part A
below.
1
metal casing surrounding
light bulb
collimating lens (to concentrate
light rays so that they travel
in straight lines)
lead to electrical outlet
slit plate with single
and triple slits
slot for color filter
Purchased light box
Time required
30 minutes for Part A
30 minutes for Part B
30 minutes for Part C
Materials
NB: Part A materials are only required for students constructing their own light
box.
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For Part A:
81/2 × 11 sheet of black poster board
craft knife or X-acto® knife
30 cm ruler
pencil
small, powerful flashlight such as mini Maglite®
(http://www.maglite.com/productline.asp)
double-sided tape or re-usable adhesive putty
translucent tape
scissors
cutting board
For Parts B and C:
light box minus its slit plate (the “small light source”)
60 watt light bulb with a tungsten filament (do not use a low energy bulb or a
spotlight) supported in a bulb holder (the “large light source”)
piece of white, unlined index card, about 3 cm high, cut to whatever shape you
like (the “object”)
4 cm sewing needle
1–2 cm of cork cut from a wine bottle cork
cardboard (e.g., from a large cereal box) about 30 cm × 40 cm to form
a “screen”
2 stands and ring clamps (to support the “screen”)
meter ruler
2 sheets of white, unlined 81/2 × 11 paper
table (at least 0.6 m wide and 1.2 m long)
translucent tape
pencil
Safety note
Cut and score the poster board in Part A on a work surface protected by a
cutting board. Do not stare directly into bright light sources. Do not touch the
light box or the light bulb when switched on.
Procedure
Perform this experiment in a room where you can restrict external light, i.e., the
windows can be covered. Begin all parts of the experiment with windows/blinds
closed and the room lights on.
Part A: Constructing a simple light box
Those with a purchased light box can omit this part of the experiment.
1. With your pencil and ruler, mark out the template shown in diagram 2 on the
next page on the piece of black poster board.
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2
single slit
top
4.5 cm
end piece
triple slit
side
5 cm
end piece
base
4.5 cm
5.5 cm
side
5 cm
overlap
Template for light box
2. Using scissors, cut along the solid lines to give two pieces of poster board. Then
cut out the two end pieces of the box from the smaller piece of poster board.
(The end pieces are slightly larger than the end of the box to allow them to be
attached firmly and thus stop light escaping around the edges of the box.)
3. Using the lines on the template in diagram 2 above as a guide, cut the slits in
the end pieces with the knife. Use the ruler to ensure that the lines are straight.
4. Using the knife and the ruler, carefully score along the dotted lines on the large
piece of poster board. Fold it to form a box and secure with translucent tape.
5. To produce a single ray of light, place the single slit end piece over one end of
the box with the open end of the slit at the base of the box. Attach firmly with
translucent tape around the sides, making sure that no light can escape around
the edges of the end piece. Your box should look like the one in diagram 3
below. To produce three parallel rays of light, use the end piece with three slits.
6. Turn on the flashlight. Put it inside the open end of the box so that it shines
toward the end piece (see diagram 4 on the next page).
7. Turn off the room lights. Adjust the position and angle of the flashlight until
the ray of light coming through the slit is seen clearly. If necessary, secure the
flashlight in place with tape or putty. Turn on the room lights.
3
front view
single slit end piece
Front view of light box
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translucent tape securing end
piece firmly to the end of box
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4
side view
flashlight
flashlight secured with double-sided tape or putty
Side view of light box
Part B: Making shadows with a small and a large light source
In this experiment the light box forms the “small light source” and the 60 watt
light bulb is the “large light source.”
1. Tape the card shape to the needle. Stick the needle into the piece of cork so
that the shape is supported upright to form the “object” whose shadow you
are studying.
2. Secure the “screen” in an upright position using the clamps and stands.
sheet of paper
taped to screen
5
clamp
“screen”
stand
card shape
light box
(”small light source”)
needle
30 cm
meter ruler
30 cm
cork
table
Arrangement of light source, object, and screen
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or transmittal is copyright protected by the publisher.
3. Place the meter rule on the table. Position the light box, the “object,” and the
screen along the rule as shown in diagram 5 on the previous page. (NB: This
shows the purchased light box.)
4. Tape one of the sheets of paper to the screen so that it is facing the “light
source,” and in line with the “light source” and the “object” (as in diagram 5
on the previous page).
5. Turn on the light and turn off the room lights.
6. Using the pencil, draw around the shadow on the piece of paper on the screen.
Label “small light source.”
7. Turn on the room lights and turn off the light.
8. Replace the small light source with the “large light source” and repeat
steps 5 to 7, labeling the shadow “large light source.”
Part C: Factors affecting the size of shadows
1. Arrange the small light source, the object, and the screen as in diagram 5 on
the previous page, but with the other piece of paper. Turn on the light source
and turn off the room lights.
2. Draw around the shadow formed on the screen, labeling the shadow
“position 1.” Turn off the light source and turn on the room lights.
3. Move the object to a point 15 cm from the light source. Turn on the light
source and turn off the room lights.
4. Repeat step 2, labeling the new shadow formed on the screen “position 2.”
5. Move the object to a position 15 cm from the screen. Turn on the light source
and turn off the room lights.
4. Repeat step 2, labeling the new shadow formed on the screen “position 3.”
Analysis
Part B: Making shadows with a large and a small light source
1. Describe the shadow formed by the small light source.
2. Describe the shadow formed by the large light source.
3. Were the two shadows the same size?
Part C: Factors affecting the size of shadows
1. Was the shadow at position 2 larger or smaller than the shadow at position 1?
2. Was the shadow at position 3 larger or smaller than the shadow at position 1?
Want to know more?
Click here to view our findings.
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10.15 • OUR FINDINGS
PHYSICS EXPERIMENTS ON FILETM
temperature
2. When there was a mixture of solid and liquid stearic acid, the temperature
rose very little (if at all). During this time, the molecules of solid stearic acid
are vibrating with increasing energy as they are heated. A point comes when
enough heat has been supplied for the molecules of solid stearic acid to have
sufficient energy to move apart and form a liquid. During this change, the
temperature remains constant even though heat is being applied. The diagram
below shows how the arrangement of the molecules changes with increasing
temperature.
melting
solid
liquid
time
Variation in temperature of stearic acid as it is heated at a constant rate
The heat required to change a substance from a solid to a liquid at its melting
point is called the latent heat of fusion. The heat required to change a substance
from a liquid to a gas at its boiling point is called the latent heat of vaporization.
Evaporation causes a liquid to form a gas at lower temperatures than at its
boiling point. The heat required to do this is called the latent heat of
evaporation. Evaporation is the reason why our skin feels cool if it becomes
damp – the dampness takes heat from our skin as it dries out. This is how
sweating helps us to keep cool on hot days.
Light
4.01 All About Shadows
Part A: Constructing a simple light box
The box needs to be long enough to allow the flashlight to be some distance
from the slit(s) forming the beam(s) of light in order to decrease the tendency for
the beam to splay out at the sides and to enable the beams produced by the triple
slit to stay parallel to each other. However, the strength of the light from the
flashlight diminishes when it is further from the slit, so a compromise may be
necessary between the strength of the beam and the amount by which it splays
out at the sides.
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PHYSICS EXPERIMENTS ON FILETM
OUR FINDINGS • 10.16
Black cardboard is used to make the box to minimize internal reflections; most
of the light reaching the front of the box thus is thus traveling in straight lines
from the flashlight. It is important that light only leaves the box through the
slit(s) in the end piece, so this must be fixed firmly onto the box.
When the flashlight is placed inside the box, some experimentation will be
necessary to find the best angle at which the flashlight should be positioned to
produce the best effect (i.e., strong beam with no divergence).
Part B: Making shadows with a large and a small light source
1. The shadow formed by the small light source should be a dark shape with
clearly defined edges (see the diagram below).
light rays
small light
source
object
screen
darker area
of shadow
of object
Shadow formed using small light source
2. The shadow formed by the large light source should be a gray shape (or even
appear as a series of gray shapes) with blurred edges (see the diagram below).
3. The entire shape of the blurred shadow from the large light source (the outer
edges are very pale) should be slightly larger than the crisp shadow formed by
the small light source. (This is not easy to see; it helps if the room is as dark as
possible.) This occurs because the light is illuminating the object from a larger
area as shown in the diagram on the next page; overlapping images are
formed by light from various parts of the light source.
screen
large light
source
object
paler area of shadow
darker area of shadow
paler area of shadow
Shadow formed using large light source
Shadows formed by the Sun
Although the Sun is a very large light source, it is so far away from the Earth
that the light rays reaching Earth are considered to be traveling in parallel, rather
than diverging from a point. If you look at the shadows formed by the object
outside in the sunshine (see the diagram on the next page), you will see that these
shadows can have sharp edges (they become more blurred as the object is moved
away from the screen).
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10.17 • OUR FINDINGS
PHYSICS EXPERIMENTS ON FILETM
object
shadow
parallel rays of light
from the sun
screen
Shadow formed by parallel rays of light from the sun
Part C: Factors affecting the size of shadows
1. The shadow formed at position 2 is larger than the one formed at position 1.
As the object moves closer to a light source, it intercepts more of the light
emitted from the source.
2. The shadow formed at position 3 is smaller than that formed at position 1
because an object further from a light source intercepts less of the light emitted
from the source. If you repeat Part C in sunlight, you will see that the shadow
does not change in size. Instead, it becomes more blurred as the object is
moved further from the screen.
4.02 Pictures In Mirrors
Part A: The laws of reflection
1. The first law of reflection states that, on reflection at the surface of a mirror,
the angle of incidence is equal to the angle of reflection.
2. The path of the incident and reflected rays could be seen on the paper, i.e.,
they were always in the same plane. This is the same plane on which the
normal was drawn. The second law of reflection states that the incident ray,
the reflected ray, and the normal to the mirror are always in the same plane.
Part B: Investigating the properties of curved mirrors
Curved mirrors form part of a sphere of a particular radius. The center of the
sphere is the “center of curvature” (C) of the mirror. The radius of the sphere is
the “radius of curvature” (r).
1. For the concave mirror, the rays of light come together or converge when
reflected and are focused to a point. This is the focal point (F) or focus of the
mirror and it is midway between the mirror and the center of curvature (see
the diagram on the next page left). The distance between the mirror and the
focal point is the focal length (f) of the mirror. Because light rays are focused
on reflection by a concave mirror, an image seen in this type of mirror is a
“real” image – it can be projected onto a screen.
2. For the convex mirror, the rays of light spread apart or diverge when reflected
and appear to come from a point behind the mirror. This point is the focal
point (F) or focus of the mirror, and is midway between the mirror and the
center of curvature of the mirror (see the diagram on the next page right). The
distance between the mirror and the focal point is the focal length (f) of the
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