The Three Parts of a Shadow

light and shadow · geometry · space science

The umbra is darkest. The umbra is the innermost part of a shadow where no light from the source reaches at all. It's the deepest, darkest part you see in the middle of a shadow.

The penumbra is halfway lit. The penumbra is the fuzzy border around the umbra where some light gets through but not all of it. This is why shadows have soft, blurry edges instead of sharp lines.

The antumbra is the light zone. The antumbra is a special outer region where light has bent around the object so much that it's actually brighter than it would be without the shadow at all. You only see the antumbra when the object casting the shadow is smaller than the light source creating it.

Eclipses show all three parts. When the Moon passes in front of the Sun, the umbra creates a zone of total darkness, the penumbra creates a zone of partial darkness, and the antumbra creates a zone where the Moon appears smaller than the Sun behind it.

Size matters for shadows. Whether you see all three shadow parts depends on the sizes of the light source and the object blocking it. If the object is bigger than the light source, the antumbra never forms.