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The Light Stuff

  • By Karen Hartley
  • Posted 10.11.05
  • NOVA

Albert Einstein realized that the speed of light—about 186,000 miles per second—is constant whether it comes from a moving source such as a speeding car's headlights or an unmoving source such as a ceiling light. But here's the catch: The speed of light is constant only in a vacuum, a place where there's no matter, like the vast emptiness of space. Here on Earth, the speed of light can slow down. In the following image, see if you can identify what ordinary objects in your house can kick light into a lower gear.

Launch Interactive

Find out why the speed of light isn't always 186,000 miles per second.

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