Likewise, light travels at its maximum speed through a vacuum-the interstellar highway-but it slows down when it enters air, water, or some other medium. Through air, light moves a hint slower: forty miles-per-second less than its maximum speed. Through glass, light moves almost eighty-thousand miles-per-second slower. Light moves more than a hundred thousand miles-per-second slower through a diamond-less than half its speed in a vacuum. Light moves so quickly anyway, do we ever notice this change in speed? Actually, we do. When light slows down it also bends slightly. This is know as refraction. Lenses use the lower speed limit of light in glass to bend and focus the light. If light always moved at it’s maximum speed, none of our telescopes, eyeglasses-or even our eyes themselves-would ever work.