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Does Light Always Travel at the Speed of Light?

Light does not always travel at the speed of light. The velocity of light can change, based on the type of materials through which it is passing. Light will travel at the constant rate known as the speed of light if it is traveling in a vacuum or in a situation in which there are no impediments to slow the progress of that light. The slowest traveling light achieved in a controlled laboratory setting was at 38 miles per hour (61 km/h), or 17,647,768 times slower than the speed of light. This phenomenon was created by directing a laser beam into cold sodium atoms, triggering a significant reduction in light velocity.

More facts about light travel:

  • Light traveling through glass or water will lose velocity as the light photons come in contact with the other molecules that make up those substances. This activity causes the photons to scatter, slowing the forward movement.

  • The speed of light in a vacuum is a consistent 186,282 miles per second (299,792.5 km/s). The velocity of light in a vacuum usually is described as velocity c.

  • Materials that have a high level of refraction tend to slow light noticeably. In addition to water and glass, materials such as alcohol, whale oil and fused quartz are considered optically dense media that have the ability to cause light photons to scatter and slow the speed of the light.

Malcolm Tatum
By Malcolm Tatum
Malcolm Tatum, a former teleconferencing industry professional, followed his passion for trivia, research, and writing to become a full-time freelance writer. He has contributed articles to a variety of print and online publications, including WiseGeek, and his work has also been featured in poetry collections, devotional anthologies, and newspapers. When not writing, Malcolm enjoys collecting vinyl records, following minor league baseball, and cycling.
Discussion Comments
By anon331057 — On Apr 20, 2013

I used to think nothing travels faster than the speed of light, school crap? Not so if you think further. The universe is moving through space; our solar system is spinning, our planet is spinning and revolving and everything is moving away from something to something? I now believe that God puts what we need to see as we look further into infinity. Where does space end? God said I will confuse the wise not enlighten.

It's almost all theories unproven and guesses. Who cares?

Malcolm Tatum
Malcolm Tatum
Malcolm Tatum, a former teleconferencing industry professional, followed his passion for trivia, research, and writing...
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