Exploring Light and Color: How Light Travels
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Light travels in a straight line at 186,000 miles/second -- or 300,000 km/s in a vacuum. Light travels slower in a substance. When light enters a new medium, it slows down or speeds up, causing... (more info)

Light travels in a straight line at 186,000 miles/second -- or 300,000 km/s in a vacuum. Light travels slower in a substance. When light enters a new medium, it slows down or speeds up, causing refraction.

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