Shadow facts for kids

Kids Encyclopedia Facts
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Shadow of a cactus on a wall
Looking down from The Eiffel Tower, Paris 8 April 2007
Shadows of visitors to the Eiffel Tower, viewed from the first platform

A shadow is a dark (real image) area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or a reverse projection of the object blocking the light.

Point and non-point light sources

Diagram of umbra, penumbra & antumbra
Umbra, penumbra and antumbra.

A point source of light casts only a simple shadow, called an "umbra". For a non-point or "extended" source of light, the shadow is divided into the umbra, penumbra and antumbra. The wider the light source, the more blurred the shadow becomes. If two penumbras overlap, the shadows appear to attract and merge. This is known as the Shadow blister effect.

The outlines of the shadow zones can be found by tracing the rays of light emitted by the outermost regions of the extended light source. The umbra region does not receive any direct light from any part of the light source, and is the darkest. A viewer located in the umbra region cannot directly see any part of the light source.

By contrast, the penumbra is illuminated by some parts of the light source, giving it an intermediate level of light intensity. A viewer located in the penumbra region will see the light source, but it is partially blocked by the object casting the shadow.

If there is more than one light source, there will be several shadows, with the overlapping parts darker, and various combinations of brightnesses or even colors. The more diffuse the lighting is, the softer and more indistinct the shadow outlines become, until they disappear. The lighting of an overcast sky produces few visible shadows.

The absence of diffusing atmospheric effects in the vacuum of outer space produces shadows that are stark and sharply delineated by high-contrast boundaries between light and dark.

For a person or object touching the surface where the shadow is projected (e.g. a person standing on the ground, or a pole in the ground) the shadows converge at the point of contact.

A shadow shows, apart from distortion, the same image as the silhouette when looking at the object from the sun-side, hence the mirror image of the silhouette seen from the other side.

Astronomy

Three moons and their shadows parade across Jupiter
Three moons (Callisto, Europa and Io) and their shadows parade across Jupiter.

The names umbra, penumbra and antumbra are often used for the shadows cast by astronomical objects, though they are sometimes used to describe levels of darkness, such as in sunspots. An astronomical object casts human-visible shadows when its apparent magnitude is equal or lower than -4. The only astronomical objects able to project visible shadows onto Earth are the Sun, the Moon, and in the right conditions, Venus or Jupiter. Night is caused by the hemisphere of a planet facing its orbital star blocking its sunlight.

A shadow cast by the Earth onto the Moon is a lunar eclipse. Conversely, a shadow cast by the Moon onto the Earth is a solar eclipse.

Daytime variation

People shadows at sunset
People with long shadows at sunset

The sun casts shadows which change dramatically through the day. The length of a shadow cast on the ground is proportional to the cotangent of the sun's elevation angle—its angle θ relative to the horizon. Near sunrise and sunset, when θ = 0° and cot(θ) = ∞, shadows can be extremely long. If the sun passes directly overhead, then θ = 90°, cot(θ) = 0, and shadows are cast directly underneath objects.

Such variations have long aided travellers during their travels, especially in barren regions such as the Arabian Desert.

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Shadow Facts for Kids. Kiddle Encyclopedia.