Light is fast, but not instant — and every distant thing you see is a snapshot of its past. Use the destination selector below to send light to the Moon, the Sun or Mars and read back the travel time from time = distance / c, then see how n = c/v slows it in a medium.
Every distant thing you look at is a snapshot of its past. The porch light next door reaches your eyes almost instantly, but the Sun you glance at is already eight minutes old, and a star 100 light-years away shows you the year 1926, not tonight. That delay exists because light, fast as it is, still takes real time to cross space. In a vacuum it travels at c = 2.998 × 10^8 m/s, roughly 300,000 km/s — the same measured value for every observer and the ultimate cosmic speed limit that nothing carrying mass can reach.
This simulation lets you pick a target with the “Choose a destination” selector — the Moon, the Sun, or Mars — and read back the light-travel time, found from time = distance / c. Light needs about 1.3 s to reach the Moon and 8 minutes 20 s to arrive from the Sun. Watch the trap here: a light-year measures distance, how far light goes in one year, and never a stretch of time.
The panel also shows the value of c and the relation n = c/v. Inside a medium, light slows to v = c/n, where the refractive index n is at least 1. It stays quickest in a vacuum and drags in glass or water, and that slowdown is exactly what bends a ray when it refracts. Test the numbers with the refractive index calculator, then try more interactive physics demonstrations.
In a vacuum light travels at c = 2.998 x 10^8 m/s, about 300,000 km/s. It is the same for every observer and is the fastest anything can travel; nothing with mass can reach it.
Because light takes time to reach us. The Sun we see is about 8 minutes old, and a star 100 light-years away shows light that left it 100 years ago, so we see it as it was then.
Distance. A light-year is how far light travels in one year, roughly 9.5 trillion kilometres. It measures length, not time, despite the word year in its name.
Its speed drops to v = c/n, where n is the refractive index of the medium and is at least 1. Light is fastest in a vacuum and slower in denser media, and that change of speed is what bends a ray when it refracts.