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Tutorial 164 - Umbra and Penumbra

Added 2016-06-29 19:51:13 +0000 UTC

Here's a visual explanation of how varying sizes of light will produce different shadows. Up to three shadow regions can be distinguished.

A point light is a single point in space. Light coming from a point light creates an umbra shadow region only.

A non-point light which has a measurable size will always create at least two regions of shadow. The umbra (darkest region) and the penumbra, a lighter shadow region.

The penumbra region is present when the light source is only partially occluded.

If we observe the moon obscuring the sun and we are located inside the penumbra region, then we would see the moon obscuring a portion of the sun. This is a parial eclipse.

The penumbra can be observed with most light setups.

For larger, non-point light sources, we see the umbra region diminish in size. The antumbra is the region that extends beyond the umbra. Similar to the penumbra, this region is partial shadow.

If we observe the moon obscuring the sun and we are located inside the antumbra region, then we should see a ring around the moon. This is an annular eclipse.

The antumbra is hard to observe as larger light sources make softer shadows.