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Science class online
Chemistry and physics lessons
atom physics lesson

Optics

Free online optic lesson for elementary school,  middle school and high school.
chemistry high school
 



















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Propagation of light

Speed of light

   
At human scale, light seems to propagate instantly from one point to another nevertheless the speed of light is not infinite.
This speed is really huge for our human senses and scientifics had been unable to measure it for a long time.

In emptiness the speed of light has the following approximate value:
300 000 km/s
or 300 000 000 m/s
or 
3 x 108 m/s

This speed has roughly the same value in the air but is lower in water (
225 000 km/s ) and in glass ( 200 000 km/s ).
More generally speed of light is lower in transparent mediums than in emptiness or air.
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