SHADOW INQUIRY
Guide to Shadow Concepts
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There are five major concepts in light and shadow. These concepts can be used to
predict or explain a wide variety of shadow phenomena. In this guide we will give some
simple demonstrations of phenomena that illustrate the five major concepts. We then
give examples of how to apply these concepts to explain a number of observations.
The five concepts are:
1) Shadow are formed by blocking (or diverting) all or part of the light hitting
a surface
2) Light travels in straight lines
3) Light travels out in all directions from the source
4) Light travels out from every point on the source
5) The more light that hits a surface the brighter that surface. Conversely, the
less light that hits a surface the darker or grayer the surface.
1) Concept 1 is illustrated after 2) and 3).
2) Light travels in straight lines
The notion that light travels in infinitely thin, straight lines is an abstraction of the real
world. However, there are a number of phenomena that suggest that this is a good
model for describing the path of light. The following give some of these examples.
Imagine holding a "point source" of light, such as a "Mini Maglite" with its top removed,
in the middle of a room. If you hold up an opaque piece of paper, it blocks some of the
light and cast a shadow on the wall. If you draw a straight line from light to the edge of
the paper and then extend this line to the wall, it will hit the edge of the shadow (Figure
1).
Figure 1
SHADOW INQUIRY
Guide to Shadow Concepts
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The phenomena illustrated in Figure 2 also suggest that light travels in a straight line. If
you stand in the shadow looking in the direction of the light you will not see the light. If
you move your eye sideways to the point where you just begin to see the light, your eye
will be on the line connecting the light, and the edge of the paper.
Figure 2
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Guide to Shadow Concepts
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Finally, Figure 3 shows that if you poke a hole in the middle of the paper, you will get a
spot of light in the midst of the shadow. If you draw a straight line from the light source
to the hole and extend that line to the wall it will hit the wall right at the spot of light.
Figure 3
These three examples all suggest that the path of light is well described by a straight
line.
3) Light travels out in all directions from the source
This concept is well demonstrated by the use of a very sophisticated light detector; your
eye. If you can see a light source, that means light from that source is entering your
eye. Imagine again, holding a "point source" of light, such as a "Mini Maglite" with its top
removed, in the middle of a room. If you ask, “”Who can see the light?” you get
responses from people on all sides. If you hold the light high, people can see the light
from below. If you stand over the light, you can see it from above. This indicates that
light comes out from the source in all directions.
Light
Source
Figure 4
1) Shadows are formed by blocking (or diverting) all or part of the light hitting a
surface.
SHADOW INQUIRY
Guide to Shadow Concepts
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Figure 5
With the previous two concepts in place, it is easy to illustrate the formation of a
shadow.
Figure 5 shows that a shadow is formed by blocking some of the light coming out from a
light source. Straight lines called rays can illustrate the path of the light coming from the
source. Where these rays strike the screen, it is bright. Points on the screen where the
light is blocked are dark. This dark area is the shadow.
SHADOW INQUIRY
Guide to Shadow Concepts
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4) Light travels out from every point on the source; and
5) The more light that hits a surface the brighter that surface. Conversely, the less light
that hits a surface the darker or grayer the surface.
These two concepts are often used together. Take an extended light source (such as a
20” fluorescent light) and cover all but a small portion at a time with your hand or a
piece of opaque paper or card. Now cover a different portion. Whatever point you leave
uncovered puts light on your screen. If you leave more points uncovered, more light
gets to the screen and the screen looks brighter.
These few concepts can explain most of the shadow phenomena that we observe.
For instance, you can use concepts 1 (shadows are formed by blocking all or part
of
the light hitting a surface), 2 (light travels in straight lines), and 3 (light travels
out in all directions from the source) to explain size and shape of shadows. If you
have a small light source (such as a “maglight bulb” and an opaque block set a fixed
distance from the light, the size and shape of the shadow will depend on the position
and orientation of the screen. In Figure 6, the entire gray area to the right of the block
has no light getting to it. When the screen is placed in position A, the shadow is
relatively small.
When the screen is placed in position B, it intersects the diverging light rays that define
the boundry of the shadow further out, making a larger shadow. Screen C is tilted and
block
light
source
screen C
screen A
screen B
Figure 6
intercepts the boundry light rays at two different widths creating an asymmetric shadow.
SHADOW INQUIRY
Guide to Shadow Concepts
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Moving the light closer to the widens the angle between the “boundry rays” and shows
why the shadow gets bigger. (Figure 7)
Screen C
block
light source
Screen A
Screen B
Figure 7
Similarly you can move the block and show what happens to the predicted shadow size.
SHADOW INQUIRY
Guide to Shadow Concepts
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By using concept 5 (the more light that hits a surface the brighter that surface),
along with the other concepts, you can explain multiple shadow phenomena. In Figure 8
we show two light sources and some of the light rays that come from those sources.
The rays from source 1 that graze the edge of the block define a widening cone of
darkness (light gray below) going downward. Similarly, rays from source 2 that graze
the edge of the block define a widening cone of darkness (dark gray below) going
upward. Where these cones overlap, light from both sources is blocked. (In Figure 8,
this is at C.) At points where only one of the cones hits the screen, light from one source
is blocked and light from the other source hits that point. We see a shade of gray. (In
Figure 8, point A illustrates this. Light from source 1 is blocked but light from source 2
can get to that point.) All other points get light from both sources and appear bright.
(Point B in Figure 8.) Note that if you move the screen further back from the light
(screen 2, the dotted line below), you will reach an area when you have one cone hitting
on top, one below and a separation in the center. Where the cones hit the screen you
will get a gray shadow. The rest of the screen is hit by light from both light sources and
will appear bright.
screen 1
light source 1
block
C
A
light source 2
B
meterstick
Figure 8
screen 2
SHADOW INQUIRY
Guide to Shadow Concepts
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Extended light sources give “fuzzy” edged shadows. Applying concept 4 (light travels
out from every point on the source) can help us to understand this phenomena. In
Figure 9, light comes from every point on the source (filament.) At point a on the screen,
light from the entire filament gets to the screen and we see that part of the screen as
bright.
a
extended light source
filament
block
screen
Figure 9a
At point b on the screen, light from part of the filament hits the screen and the rest of the
light is blocked. We see a shade of gray at that point. This part of the shadow, where
some but not all of the light is blocked, is called the penumbra.
screen
block
b
filament
Figure 9b
SHADOW INQUIRY
Guide to Shadow Concepts
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At point c on the screen, all of the light is blocked. We see black on the screen. In
Figure 9c
The dashed lines represent rays from the end points of the filament which define the
edge of the area where the light is totally blocked. This part of the shadow, where all of
the light is blocked, is called the umbra.
a
b
filament
c
block
screen
Figure 9c
As you move on the screen from point c, to point b, to point a, more and more points of
the filament shine on that part of thle screen. The shadow gradually goes from black to
lighter and lighter shades until there is no shadow at all at point a. This gradual change
makes the edges of the shadow look fuzzy.
By applying our few basic concept as we did in these example, almost all of the shadow
phenomena that you encounter can be explained.
Exploratorium Institute for Inquiry