Speed of light in water
| Value | 224,900,000 m/s |
| Status | Measured: ± 90,000 m/s (0.0004 relative) |
| Source | Derived from n = 1.3330 (typical, 589 nm, 20 °C) |
| Categories | Electromagneticopticsphysics |
| meter per second | 224,900,000 m/s |
| kilometer per hour | 809,640,000 km/h |
| foot per second | 737,860,890 ft/s |
| mile per hour | 503,086,970 mph |
| knot | 437,170,630 kn |
| foot per minute | 44,271,654,000 ft/min |
| centimeter per second | 22,490,000,000 cm/s |
| meter per hour | 809,640,000,000 m/h |
| meter per day | 19,431,360,000,000 m/d |
| foot per day | 63,751,181,000,000 ft/d |
Learning zone
Water's refractive index at the sodium D line and 20 °C is 1.3330, so light crawls through it at three-quarters of its vacuum speed. Léon Foucault measured exactly this in 1850, showing light is slower in water than in air, and in doing so killed Newton's corpuscular theory, which required the opposite. The slowing is dispersive — blue light is slowed slightly more than red, index 1.3403 at 434 nm against 1.3311 at 656 nm — which is why a rainbow exists at all.
Because c/n is less than c, charged particles can legally exceed the local speed of light in water without troubling relativity. When they do they leave a blue optical shock wave, Cherenkov radiation, and vast water tanks like Super-Kamiokande use it to see neutrino interactions. The value quoted here is a typical figure for pure water in the visible band; it varies with wavelength, temperature and salinity.