Light Slowed to 70 Km/h: Causes & Effects

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I understand that the speed of light in different mediums (like water) is slower than that of light in a vaccume. I've also read about some expiriments done in which light had been slowed down to a quite imaginable speed, like 70 Km/h. What causes light to slow down in such instances? Is it just bouncing around a lot, or do the photons actually change speed? If the photons chance speed, WHY?!
 
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You may want to start by reading an entry in the FAQ in the General Physics forum.

Zz.
 
Ah! Sorry, I'll check that first next time.
Thank you for your help.
 
Nevertheless A.I. welcome to these Forums and keep asking the questions! :smile:

Garth
 

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