What Sound Needs

Watch a thunderstorm from far away and you'll notice something strange. The lightning flashes — and then you wait. One second, two, five, maybe more. Only then does the thunder arrive.

Both were made at the same instant, by the same bolt. So why do they turn up at different times?

Because light is enormously faster than sound. The flash reaches you almost instantly; the bang takes its time. Nothing else is different — the delay is purely the two of them travelling at different speeds, which is exactly the idea from lesson 4 showing up in the sky.

But there's a bigger difference between light and sound, and it's stranger than the speed. Light crossed empty space to get here from the Sun. Sound can't do that at all. Sound cannot travel through nothing. It needs something to travel through — and if you take that something away, sound simply stops existing.

By the end of this lesson you'll be able to:

• explain how every sound is made
• explain why sound cannot travel through a vacuum, and why light can
• tell the difference between a loud sound and a high sound
• explain what an echo is and why thunder lags behind lightning