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A stork casting a shadowA stork casting a shadow When light falls on a substance that it can pass straight through, for example, clear glass or water, that substance is said to be transparent. Materials that do not allow light to pass through them, such as wood or metal, are described as opaque. When light falls on an opaque object, a shadow forms on the opposite side of it, because light can only travel in straight lines. The most spectacular example of a shadow is a solar eclipse—the shadow cast by the Moon on the Earth when it blocks the light from the Sun

Umbra and penumbra in a lunar eclipseUmbra and penumbra in a lunar eclipse

Umbra and penumbra

A small, concentrated beam—such as from the Sun—casts a sharp-edged, dark shadow, When light comes from a larger source, such as a fluorescent tube, or from several different directions, the shadows cast are not as dark and have fuzzy edges. Shadows have two parts: the umbra is the dark region where all the light is blocked; the penumbra is the less dark region in which only part of it is blocked.


Length and direction

The earliest sundials that we know are the obelisks, tall, tapering stone towers with pyramid-like tops built in ancient Egypt from about 3500 BC.

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