Understanding light
Time of flight
Light crosses a 300 mm bench in one nanosecond. Slow that down two billion times and it becomes a thing you can watch travel — which is exactly what Photonica's light-in-flight mode does.
How fast, exactly
In vacuum, light travels at 299,792,458 m/s — about 300 mm per nanosecond, or 0.3 mm per picosecond. In glass it goes slower, and for a pulse the relevant number is not the plain refractive index but the group index ng, which accounts for dispersion:
Light in flight
In the Waves panel, turn on Show pulse. A packet of light leaves the source and crawls across the bench. The controls:
| Control | Does |
|---|---|
| Playback speed | Picoseconds of simulated time per second of real time — the slow-down factor |
| Pulse length | How long the pulse is. Real femtosecond pulses are under a micrometre; a longer one is simply easier to see |
| Screens | What the screens show while the pulse runs: everything, arrived so far (a photon counts once its travel time has passed), or only while it hits — like a streak camera |
| Auto range | Loops from zero until the beam reaches its last surface. Turn it off to set the start and end yourself |
Beyond the bench
View → Planet-scale light travel opens a calculator that uses the same arithmetic on much longer distances: between cities on the WGS84 ellipsoid, through optical fibre (ng = 1.4682, with a route factor because cables never run straight), via geostationary or low-Earth satellites, and out to the Moon, Mars, Voyager 1 and Proxima Centauri.
It is the fastest way to feel why a video call to the other side of the planet has a floor on its latency that no amount of engineering can remove.