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.
Let's look at some exploratory activities using shadows that may be used to introduce geometrical optics and demonstrate applications of shadows in perception and the visual arts.
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, you 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 partial eclipse.
2001 Solar Eclipse Composite by Wendy Carlos, Williams College.
A Challenge for your Students:
The 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.
The 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?
2. Default 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.
Image 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
3. Shadows in the Arts
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 drawing?
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, around 1637
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: Drawing Hands 1948
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.
4. 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 convex.
To observe this reversal, cut an L-shaped piece of paper consisting of three segments about two inches square. 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.