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Light takes time

Space and time · level 3, lesson 1 of 6

A question to start

In a storm, you see the flash of lightning first, and hear the thunder afterwards. But both happen at the same moment. Why do they reach you at different times?

The idea

Light and sound both take time to travel, but light is far, far faster.

  • Sound travels about 343 metres each second in air. It takes about 3 seconds to go 1 km.
  • Light travels about 300,000 kilometres each second. Across a few kilometres, it arrives almost at once.

So the flash reaches you first, and the thunder catches up later. Count the seconds between them and divide by 3: that is roughly how many kilometres away the lightning struck.

For a long time people thought light arrived instantly. In 1676, Ole Rømer, a Danish astronomer, noticed that Jupiter's moon Io seemed to run late when Jupiter was far from Earth, and early when it was close. Light, he realised, was taking longer to cross the bigger distance.

light: almost at oncesound: about 9 seconds3 km

Worked examples

1. Counting. You count 6 seconds between the flash and the thunder. \(6 \div 3 = 2\): the lightning was about 2 km away.

2. Sound over 1 km. \(1{,}000 \div 343 \approx 3\) seconds.

3. Light over 1 km. \(1 \div 300{,}000\) of a second: about three millionths of a second.

Try it

  1. Which travels faster, light or sound?
  2. About how far does sound go in a second?
  3. You count 9 seconds between flash and thunder. How far away was the lightning?
  4. What did Ole Rømer find out about light?
Answers
  1. Light.
  2. About 343 metres.
  3. \(9 \div 3 = 3\) km.
  4. That it takes time to travel: it is fast, but not instant.

Common mistake

"Thunder comes after lightning because it happens later." Both happen at once. The thunder is simply slower to reach you.

Where it leads