- Do you hear thunder and see lightning at the same time?
- Do you see light as soon as a distant lamp is lighted?
- How long does a sound take to travel through the air?
- Do soldiers on the march all, hear a band together?
You have noticed, perhaps, during a thunderstorm, that the clap of thunder is heard after the flash of lightning is seen. You may have noticed, too, if you have stood at some distance from a railway station as a train steams out, that the sound of the engine’s whistle reaches you some seconds after you have seen the puff of steam. The thunder-clap and the lightning flash, the whistle and the puff of steam, really take place at the same instant. How is it, then, that the one is heard after the other is seen?
The explanation is simply this: that sound travels much less quickly than light, or, in other words, that the vibrations which produce the sensation of light are much more rapid than those which produce the sensation of sound. When the lanterns of a lighthouse are lit; miles away, we see the light practically at the very moment of lighting. But the movements in the air which I have already described take some time. It is found that it takes a second of time for sound to travel 1100 feet; so that if you are standing at a distance of 1100 feet from a railway engine when it whistles, you will hear the shriek of the whistle one second after you see the puff-provided the air is still. If a strong wind is blowing in the opposite direction, you may not hear the sound at all.
Can you see now how it is possible to tell how far off a thunder-cloud is when it bursts? Suppose what you hear the thunderclap exactly three seconds after you see the flash; you know that sound travels 1100 feet in one second, so it must have travelled three times that distance in three seconds, and thus you see that the cloud must have been 3300 feet from your ear.
You will see also how it is that a large body of soldiers, marching to the music of a band ahead, can never keep absolutely in step. The rear-rank men hear the band of the drums and the tira-lira ci the trumpets some time after the front-rank men heard them, and consequently the feet are not lifted quite at the same instant. The longer the marching troop is, the more will the rear ranks be out of step with the front ranks.