What is umbra?

An umbra is the darkest part of a shadow that happens when something blocks all the light.

Imagine you're playing with your favorite toy flashlight in a dark room. When you turn it on, the light shines out and makes a bright circle on the wall. Now, if you put your hand between the flashlight and the wall, you’ll see a shadow, the part where the light can't reach. That shadow is like an umbra, but only if your hand completely blocks all the light.

Think about a full moon eclipse. The Earth moves between the Sun and the Moon, blocking the sunlight that normally shines on the Moon. If the Earth blocks all the sunlight, the Moon goes dark, just like how your hand makes the wall go dark in the shadow game. That dark part is the umbra.

Sometimes, if only part of the light is blocked, you get a lighter shadow called a penumbra. But the umbra is the full-on, total shadow, like when your hand is right in front of the flashlight, and it’s completely blocking all the light.