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Lesson: Chapter 4 — Waves and their applications

4.3.2. The speed of sound 1 of 2

Lightning before thunder

In a nearby thunderstorm you see the flash of lightning first, and hear the thunder a little later — even though both happen at the same moment.

Figure to be added

Figure 4.27 — lightning over a lake at night.

The light from the lightning travels to us, and we see the flash when it enters our eyes. Light travels at 300 000 km s-1 (3 × 108 m s-1), so it reaches us almost at once.

The thunder arrives a little later because sound takes much longer than light to cover the distance from the lightning to us.

How far away was it?

Count the seconds between the flash and the thunder. Sound covers about 330 m each second, so a 3-second gap means the lightning struck about 1 km away.