Explore how shadows form and why they have soft edges. With a small light source shadows are sharp; with a large one, a soft outer shadow (penumbra) surrounds the dark core (umbra).
A point light source creates a sharp umbra. An extended source creates a penumbra — the region where only part of the source is blocked. This explains partial solar eclipses, soft photographic shadows, and why shadows are sharper on sunny than cloudy days.
Drag objects between the light source and the wall. Change light source size to see the penumbra grow. Add multiple light sources for complex overlapping shadows.
During a total solar eclipse, the Moon's umbra is typically only 160 km wide on Earth's surface. Standing exactly in the umbra gives total darkness at midday; one step into the penumbra restores partial sunlight.