Table of Contents
What Is the Speed of Light?
The speed of light in vacuum, denoted c, is exactly 299,792,458 meters per second. Since 1983, this value defines the meter: one meter is the distance light travels in 1/299,792,458 of a second. The speed of light is the ultimate speed limit in the universe according to Einstein's special relativity—no object with mass can reach or exceed it.
Light slows down when passing through transparent materials like water, glass, or diamond. The ratio of c to the speed in a medium is called the refractive index (n). In water (n = 1.33), light travels at about 225 million m/s. In diamond (n = 2.42), light moves at only about 124 million m/s, which is what gives diamonds their brilliant sparkle through total internal reflection.
Formulas & Constants
Where n is the refractive index of the medium, d is distance, and t is time. One light-year equals 9.461 × 10^12 km.
Speed in Different Media
| Medium | Refractive Index | Speed (m/s) |
|---|---|---|
| Vacuum | 1.0000 | 299,792,458 |
| Air | 1.0003 | 299,702,547 |
| Water | 1.33 | 225,407,863 |
| Glass | 1.52 | 197,232,538 |
| Diamond | 2.42 | 123,882,834 |
Cosmic Distances
| Journey | Light Travel Time |
|---|---|
| Moon to Earth | 1.28 seconds |
| Sun to Earth | 8.3 minutes |
| Sun to Mars | 12.7 minutes |
| Sun to Jupiter | 43.3 minutes |
| Nearest star (Proxima Centauri) | 4.24 years |
| Across Milky Way | ~100,000 years |
Frequently Asked Questions
Can anything travel faster than light?
According to special relativity, no object with mass can reach or exceed the speed of light. As an object accelerates toward c, its relativistic mass increases toward infinity, requiring infinite energy. However, the expansion of space itself can cause distant galaxies to recede faster than c, and quantum entanglement correlations appear instantaneous but cannot transmit information.
Why does light slow down in glass?
Light interacts with the electrons in the glass material. The photons are absorbed and re-emitted by atoms, and the delays from these interactions effectively reduce the overall speed. The individual photons still travel at c between atoms, but the net propagation speed is lower.
How was the speed of light first measured?
Ole Roemer first estimated the speed of light in 1676 by observing timing variations in the eclipses of Jupiter's moon Io. More precise measurements followed from Fizeau's rotating toothed wheel (1849) and Foucault's rotating mirror (1862). Modern laser interferometry gives extremely precise values.