Shadows form when light shines upon an object. While this is a simple concept, the shadow volume behind the object can reveal startling phenomena. For example, a lunar eclipse occurs only when the moon is full and when it passes through part of the Earth’s shadow. Physicists describe shadow volume in terms of umbra and penumbra.
TL;DR (Too Long; Didn't Read)
The umbra is the darkest part of the shadow while the penumbra is the lighter portion on the edges.
Root of Origin
Umbra and penumbra are Latin. The term “umbra” means shadow. The “pen” in penumbra derives from the Latin “pendere,” which means to hang. The penumbra “hangs” on the umbra; the penumbra is the lighter shading that results when a light source shines on an object.
In physics, the umbra refers to the darkest part of the shadow. If you aim a light source directly at an object, the blackest portion of the shadow directly behind the object is the umbra of the shadow. In astronomy, if the sun is shining directly on the western hemisphere of the earth, the eastern hemisphere is bathed in darkness, and objects behind the earth (such as the moon or other planets) are in the earth’s umbra.
Penumbra refers to the lighter shading that hangs on the outskirts of the umbra. The penumbra is not a true shadow. The Lighting Design and Simulation Knowledgebase describes the penumbra as a gradient: The shading gradually lessens from shadow to light as the penumbra stretches away from the umbra.
Application: Lunar Eclipse
A lunar eclipse occurs when the moon passes through the earth’s shadow. There are three types of lunar eclipses. A penumbral lunar eclipse occurs when the moon passes through the earth’s penumbra. According to MrEclipse.com, penumbral eclipses are difficult to observe. The other two types of eclipses are partial and total eclipses. Both occur when the moon passes through the earth’s umbra.
Forming an Umbra and Penumbra
You can experiment with umbras and penumbras at home. Find a blank wall, and position a light source 6 to 10 ten feet away from it. Turn the light on, and stand between the light source and the wall. Take note of your shadow on the wall. As you move closer to the wall, the darkest part of the shadow -- the umbra -- darkens and the partial shadow -- the umbra --- begins to fade more fully into the umbra. As you move away from the wall, your shadow grows and the umbra gradually gives way to the penumbra.