What Is a Penumbra?

When an opaque object blocks an extended light source (not a point source), three shadow zones form: the umbra (full shadow, where no direct light reaches), the penumbra (partial shadow, where some but not all of the source is visible), and sometimes an antumbra (beyond the tip of the umbra cone, where the obscuring object appears smaller than the source). During a solar eclipse, observers in the penumbra see a partial eclipse, while those in the narrow umbral path see totality. Sunspots also have a penumbra — the lighter, striated region surrounding the dark core (umbra) of the spot. The penumbral concept applies to all wave phenomena, including sound shadows behind buildings.