The umbra is the innermost and darkest part of a shadow
The penumbra is the region in which only a portion of the light source is obscured by the occluding body.

1 5 1
UMBRA--------> The umbra is the area of the shadow where all the light from the source has been blocked, preventing any light from falling on the surface. ................... PENUMBRA----------> The penumbra, or partial shadow, is a section where light from only part of the source is blocked, resulting in an area where the light is dimmed, but not totally absent.
1 5 1