If, as is extremely likely, you have ever witnessed a thunderstorm, you will have noticed the time between seeing the lightning and hearing the thunderclap. The lightning and thunderclap happen at the same time, so why the difference?
The difference is due to the method by which they reach us. Light travels very fast – 300000000 m/s, but the speed is sound is much less – 330m/s, or only
the speed. The light will reach us almost instantaneously, but if for example, the thunderstorm is 2km away, the thunderclap will take about
This can work the other way around. If you can count the seconds between the lightning flash and the thunderclap, then you can estimate roughly how far away the thunderstorm is by assuming that for every one second the thunderclap has travelled 330 m, so you can work out the distance using distance=speed * time=330t