There’s a saying that goes “The speed of light is greater than the speed of sound, which is why some people appear bright before they open their mouths”. Whilst I’m sure that we can all remember someone who fits that description exactly not many people appreciate just how vast the difference is between the speed of light really is. Indeed in everyday life you can pretty much consider light to travel instantaneously since it could reach any point on earth in under a millisecond. That also means that visually observed phenomena can help us determine other things, like how far away the boat in the below video was from the volcano that erupted:
From the first point where you can see the eruption beginning to the time when the shockwave hits the camera approximately 13 seconds elapses. Taking into account that the speed of sound in air (roughly 341 m/s, although it could be slightly faster depending on the temperature) that gives us an approximate distance of 4.4 km from the eruption site. To put that in perspective the light that brought the picture traveled the same distance in about 0.01 milliseconds, an imperceptibly short amount of time. If you were so inclined you could also figure out all sorts of other kinds of information from this video (like the height of the plume, it’s velocity, etc.) but they’re an exercise I’ll leave up to the reader.
This video also showcases one of the coolest (in my opinion) visual phenomena related to massive explosions like this. You can see the shock wave propagating out from the epicenter very clearly, something which always happens but isn’t usually visible to the naked eye. Here you can see it travelling outwards thanks to it compressing the air in front of it which changes the refractive index of light. With explosions of this magnitude the amount of compression, and the resulting shock wave, are enough to produce a significant bend in the light passing through it.
I probably wouldn’t want to be that close to the explosion though!
