penumbra — the partially shaded outer region of the shadow cast by an opaque object

a teacher holds up a large cardboard circle to demonstrate eclipse shadows while children gather around. The cardboard creates a complete dark shadow in the center where no sunlight passes through. Around the edges of the main shadow, there are lighter gray areas where sunlight is only partially blocked. The children point excitedly at the different shadow zones, comparing the completely dark center to the lighter outer regions. Some kids hold their hands in the lighter shadow area, noticing how it's dimmer than full sunlight but brighter than the complete shadow. the partially shaded outer region of the shadow cast by an opaque object