Light travels at different speeds in different materials. In air, or the vacuum of space, it travels very fast (about 299,792,458 meters per second). In water, light travels 1 and one third slower than it does in air. We say water has a refractive index of 1.33.
Material Index
Quartz   1.54
Salt   1.54
Mineral oil   1.48
Corn oil   1.47
Glycerine   1.47
Ethanol   1.36
Water   1.33
Ice   1.31
Air   1.00
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