Speed of light in glass
| Value | 197,230,000 m/s |
| Status | Measured: ± 5,000,000 m/s (0.025 relative) |
| Source | Derived from n = 1.52 (typical crown glass, visible band) |
| Categories | Electromagneticopticsphysics |
| meter per second | 197,230,000 m/s |
| kilometer per hour | 710,028,000 km/h |
| foot per second | 647,080,050 ft/s |
| mile per hour | 441,190,940 mph |
| knot | 383,384,450 kn |
| foot per minute | 38,824,803,000 ft/min |
| centimeter per second | 19,723,000,000 cm/s |
| meter per hour | 710,028,000,000 m/h |
| meter per day | 17,040,672,000,000 m/d |
| foot per day | 55,907,717,000,000 ft/d |
Learning zone
Ordinary crown glass has a refractive index near 1.52, so light inside it moves at about two-thirds of its vacuum speed. Every lens works by exploiting that delay: the thick centre retards the middle of a wavefront more than the thin edge, curving the wave into convergence. The index is not one number but a curve — flint glasses reach 1.6 to 1.9, fused silica sits at 1.458, and each disperses differently, which is how an achromatic doublet cancels colour fringing by pairing crown and flint elements.
In telecommunications the same slowing sets the clock. Light in a silica fibre travels at roughly 2.0×10⁸ m/s, so a signal takes about 5 μs per kilometre, and a London-to-New York round trip cannot beat about 56 ms no matter what the equipment does. That physical floor is why high-frequency trading firms paid for microwave links through the air, where the index is essentially 1. Treat the value here as typical for crown glass rather than a property of "glass" in general.