In Mark Twain’s Connecticut Yankee in King Arthur’s Court, King Arthur would have burned Hank Morgan to death on a stake if light rays did not travel in straight lines. But when the moon blocked the sun’s rays from reaching Camelot, Hank was spared from the fire and, instead, christened the principal minister to the King. Few people are spared from the stake by understanding the way light forms shadows, but many photographers are spared from dull photographs by understanding the basic principles that govern the sharpness or softness of shadows.
By thinking about light as a collection of rays that propagate through air in straight lines, we can understand how the sharpness or softness of a shadow depends on the relative size of a light source. Suppose, for example, that a small source is projecting light throughout some space so that every position in the space is illuminated by one ray of light.
Now, if we place an opaque screen in the space, then some of the rays will be blocked by the screen and a shadow will form behind the obstruction. If the screen blocks a ray from reaching a particular location, then that location will be in the shadow; if the screen does not block the ray, then the location will be in the light. The dark region behind the screen that sees no light is sometimes called the umbra, which is the Latin word for shadow.
If the source is moved to a different location in the space, then the shape of the shadow will change. Some locations that were in the light will be in the shadow, and some locations that were in the shadow will be in the light. But, because the source is small, each location will be either fully in the shadow or fully in the light.
If two small sources are in the space, then some locations will be shadowed from both, some locations will be illuminated by both, and some locations will be illuminated by one source but not the other. For the configuration shown below, for instance, locations that are below and to the right of the screen will be illuminated by the source on the right but not by the source on the left. The light in those locations will be half as bright as in locations that are illuminated by both sources. Likewise, locations that are below and to the left of the screen will see light from the left source, but be in the shadow of the right source. The regions that are partially shadowed are sometimes called the penumbra.
A large extended light source will illuminate the space from many directions, and, because of that, the screen will cast a much softer shadow.
If the source is moved closer to the screen it will appear to be larger and the screen’s shadow will be softer yet. Professional photographers understand this phenomena well, and they soften shadows by using diffusing devices like reflectors, screens, and umbrellas to make their source appear larger to the scene.
© 2011 Timothy Schulz