A shadow is a dark shape that forms when something blocks light. Light travels in straight lines, drawn as rays, and a shadow forms where an opaque object blocks light from the Sun or another source. Things that do not let any light pass through are called opaque.

A shadow in sunlight is not pure black. The object blocks the light of the Sun but not the ambient light of the sky, and such a shadow can have a bluish tinge.

The edge of a shadow depends on the light. A small, point-like source of light gives a sharp edge. The wider the light source, the more blurred the shadow's edges become. A point source casts only a simple shadow called an umbra, but a wider source, like the Sun, makes a shadow with more than one part.

An eclipse happens when an object passes into the shadow of another body. In a solar eclipse the Moon passes in front of the Sun. Inside the Moon's umbra the Sun is completely covered, and an observer sees a total solar eclipse. In the penumbra only part of the Sun is covered, so an observer sees a partial eclipse. In a lunar eclipse the Moon passes through Earth's shadow.

Words to know

• umbra: darkest shadow part
• penumbra: partial shadow part
• ambient: all-around
• eclipse: sky shadow