What is the term for the lighter part of a shadow through which the light source can be partially seen?

• A. Umbra
• B. Penumbra
• C. Shadow
• D. Antumbra

Answer: (B)

When a light source is blocked by an object, the shadow has distinct regions that reflect how much of the light gets through. The darkest, fully blocked region is the umbra. The lighter part where the light source is only partially blocked, so you can still see part of the light, is the penumbra. The farthest region, where the light surrounds the occluder, is the antumbra. So the lighter portion of the shadow through which the light source can be partially seen is penumbra. For context, during a solar eclipse, observers in the penumbra witness a partial eclipse, those in the umbra see a total eclipse, and those in the antumbra see an annular eclipse.