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Color and Light · entry 22 of 52
The darkest part of a shadow is called the umbra, while the softer edge is the penumbra.
The longer version
Umbra and penumbra are names for the two zones an extended light source creates. In the umbra the source is completely hidden, so no direct light arrives at all; in the surrounding penumbra only part of it is blocked, giving partial illumination and a soft transition.
The terms come from astronomy, where they describe solar and lunar eclipses. Standing inside the umbra during a total solar eclipse means the sun is fully covered; standing in the penumbra yields a partial one. The same vocabulary applies to any shadow cast by anything larger than a single point.
Where this entry sits
These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.
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Read 24A mirror swaps left and right in reflections, though it really reverses front and back.
Read 25Kaleidoscopes create symmetrical patterns by reflecting light between angled mirrors inside a tube.
Read 26Water bends light, which makes a straight straw in a glass look broken or shifted.
Read 27Mirages appear on hot roads when layers of air bend light from the sky toward the ground.
Read 28A rainbow is actually a full circle, but the ground usually hides the lower half.
Read 29Double rainbows happen when light reflects twice inside each droplet, reversing the color order.
Read 30You never see a rainbow from directly overhead because the geometry requires looking away from the sun.
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