Shadows are ubiquitous, but often go
unnoticed. Shadows are important historically, for they provided
early evidence that light travels in straight lines. Humans
constantly, but unconsciously, use shadows to judge the shape of
objects in their environment. Because shadows reveal much about an
object’s extension in space, they are often used to heighten the
illusion of depth in a painting.
In this edition of CoolStuff we will look at some exploratory
activities using shadows that may be used to introduce geometrical
optics and demonstrate applications of shadows in perception and the
PS: What's wrong with the above picture?
How many things can you find?
Everyone has tried making the talking duck when presented with the
opportunity of a bright light casting a shadow. Here is a creative
example of shadow puppets used in a commercial from Argentina. Just
click the image to the left to view..
1. Simple Shadows
In a darkened room, use a point source
of light to form the shadow of a small object, such as a small box
or ball, on a screen. An LED flashlight with a single bulb or
automobile tail light bulb serves as a good approximation to a point source. When
a single point source is used to illuminate an object, two distinct
regions should be observed. In one region, light from the source is
completely blocked. This region is referred to as the umbra. Outside
the umbra, light is not affected by the object.
When a second point source is introduced as shown in the figure
should observe two types of shadowing. Once again there is the
umbra, a region on the screen that is in complete darkness. In the
umbra, light from the two sources is blocked. Outside the umbra is
the penumbra where light from one bulb reaches the screen but light
from the other bulb does not.
Now replace the two point sources with what is referred to as an
extended source. An extended source may be thought of as a very
large number of point sources. A frosted light bulb and a
fluorescent tube are examples of extended sources. Describe the
shadows produced by an extended source.
To understand the shadows produced by an extended source, it may be
useful to remember that each point on the surface of the bulb,
acting as a point source, produces its own shadow. When all these
point source shadows are superimposed, there will be total darkness
(umbra) surrounded by a lighter region (penumbra) where only some of
the individual shadows overlap.
During eclipses of Sun, the moon
intervenes and casts a shadow on the earth. Observers in the umbra
see a total eclipse, while observers in the penumbra see only a
Solar Eclipse Composite by
Challenge for your Students:
earth is approximately 93 million miles from the sun. At this
great distance, can the sun be considered to be a point source
of light? A simple experiment should provide an answer.
next time you are out on a sunny day examine the shadow produced
by a pole, a leaf, your hand, or virtually any other object. How
will this observation help you answer the question?
Assumptions Regarding Shadows
Humans constantly, but unconsciously,
use shadows to judge the shape and location of objects in their
environment. In doing so, we all rely on the default assumption that
sources of light are overhead. We live in a world where light almost
always comes from above. Have you noticed how children never cease
to be delighted by the effects produced by illuminating the face
with a light from below? Why does this always bring chuckles? Humans
are simply not used to seeing the shadows formed by a light source
located beneath the face.
Sometimes this hard-wired assumption regarding light placement can
lead to incorrect conclusions regarding the nature of an object. For
example, in is this photograph we see large indentations among an
array of rivets on the hull of a ship. This percept is based on the
nature of the shadows and the assumption that the light source is
overhead. When the photo is inverted, things change dramatically!
Rivets become divots, and vice versa.
Right side up
Same image turned upside down
While students always enjoy this
demonstration, some may ask “who cares.” You may wish to point out
to them that astronauts landing on the moon care a great deal about
the actual nature of the lunar surface.
In the figure below, we see a crater.
However, when the photo is turned upside down (right), the shadows suggest
otherwise. We now see a hill.
Crater Image Upside Down
In this photograph from “Walter Wick’s Optical Tricks,” we see a
number of pieces of wood on a woodworker’s bench. The odd-shaped
pieces of wood are illuminated from above as we can see from the
shadows. When the photo is turned upside down, and the shadows
shift, we see something entirely different! Do you see the deer
surrounded by branches and leaves?
Click the image to see the movie
Demo of a page from Walter Wick's
Click here for
information on how to order "Walter Wick's Optical Tricks" book
3. Shadows in
Because shadows reveal much about an
object’s extension in space, they are one of an artist’s most potent
depth cues. Notice how a circle becomes a sphere with the addition
of shadow and shading. Where is the source of light in this
In this rather simple sketch of an
elephant by Rembrandt, the sense of depth and solidity is due
in large part to the adroit use of shadows.
Rembrandt van Rijn, An Elephant, black chalk and charcoal,
In Escher’s “Drawing Hands,” shadow and shading are used to create a
sense of three-dimensionality. The hands seem to pop right off the
piece of paper.
Escher, Maurits Cornelis:
In paintings such as “An Experiment on a Bird in the Air Pump” by
Joseph Wright, the use of strong contrasts of light and dark may be
used to discuss the nature and location of the light source as well
as the inverse square law. The rather sharp shadows suggest a point
source such as a candle. Notice too that even though the light
source cannot be seen, its location can be inferred. And perhaps most
importantly, the shadows establish the mood of the painting.
An Experiment on a Bird in an Air
Pump by Joseph Wright of Derby, 1768
The scene in Edward Hopper’s “The Night Hawks” is totally devoid of
harsh shadows. Why? When this work was created, fluorescent lights
had become commonplace. The uniform lighting produced by a
collection of extended sources does not produce sharp shadows. The
result: a mood of detachment and loneliness.
Nighthawks (1942) by Edward Hopper.
Copyright Notice: These
images are of a drawing, painting, print, or other two-dimensional
work of art, and the copyright for it is most likely owned by either
the artist who produced the image, the person who commissioned the
work, or the heirs thereof. It is believed that the use of
low-resolution images of works of art; for critical commentary on,
the work in question, the artistic genre or technique of the work of
art, qualifies as fair use under United States copyright law.
Turning Things Inside Out with Shadows
A demonstration of the power of shadows
that never ceases to amaze students involves reversed
three-dimensional figures. By manipulating the
light striking a concave object it is possible to make it appear
To observe this reversal, cut an L-shaped piece of paper consisting
of three segments about two inches square (see figure). Fold at the
two lines joining the squares and join them with transparent tape to
make half a cube.
With one eye closed, hold the concave
corner at arm’s length and orientate it so that it appears to be
convex. That is, at some orientation, the concave cube corner will
appear to reverse itself! Amazing! Once you have achieved reversal
of the corner, rotate it in your hand and notice that the cube
appears to turn in the opposite direction.
An inexpensive plastic mask may be used to illustrate the same
effect. With one eye closed, the concave side (backside) of the mask
appears convex. Furthermore, the face seems to follow you as you
move from side to side! Working in concert with the shadow cues is
our expectation to see a convex face. We have rarely seen a convex
face, so we tend to see what we believe.
Einstein Alive! You have to see it to believe it!
Shine a source of light at the back of this mask and look at the
concave side. The mask appears to reverse, as if Einstein is looking
at you! Move back and forth, and his face turns to follow you. Move
up and down, and he nods his head.
This just added!
just received these amazing photos that fit perfectly with our
topic, so we thought we would share them with you. Click the link to
see the whole series...
3D Mural Artwork
Sidewalk Artist Julian Beever
Click here to see the amazing walk...
We find it difficult to believe
that this issue of CoolStuff is our 23rd. We have
thoroughly enjoyed bringing CoolStuff to you and hope you have
found the ideas presented in the newsletter useful in your
science teaching. This year’s final CoolStuff features some of
our favorite activities from the past 23 issues. We hope our
selection is to your liking. Have a great summer!
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