Speed of sound: speed of light

Different speeds of sound and light

Thinking about the learning

Pupils need to differentiate between the speed of sound in air (about 340 metre / second) and the speed of light in air (about 3 × 10⁸ metre / second).

Teacher Tip: Introduce everyday examples to emphasise that sound and light travel at different speeds.

Thinking about the teaching

Many pupils are familiar with the situations that can be referred to in drawing attention to the difference between the speeds of sound and light. For example, they may know that you can see a flash of lightening before you hear the peal of thunder. They may not, however, be clear about how this observation can be accounted for.

We see the flash virtually instantaneously because light travels so fast. Sound, on the other hand, travels more slowly (about 300 metre / second). If the storm is one mile away – about 1500 m – the sound will take about 5 s to arrive.

Other situations that you might talk about with pupils include:

Sprinters start on the flash of a gun, rather than the bang. On school sports day, the starter actually stands half way down the 100 m track with the starting pistol. How long will it take for the bang to arrive? (About 0.3 s).