The Electromagnetic Spectrum
Light Sound and Waves

The speed of light

Teaching Guidance for 11-14 Supporting Physics Teaching

Light does not just get here in no time at all

Wrong Track: Light gets there straight away: there's no gap between the lightning and the place being lit up.

Right Lines: Light does travel very quickly: but it still takes time to make the journey from the source. The speed of light is large, not infinite.

The speed of light

Thinking about the teaching

The speed of light, in metres per second, is one of those huge numbers that is occasionally thrown up in science and that can capture pupils' interest and imagination. A good starting point to help pupils appreciate the magnitude of this speed is to consider speeds that the pupils will be able to imagine:

Teacher: Does anybody know what the world record is for the 100 metres?

Linford: It's 9.58 second!

Teacher: Spot on! Let's say about 10 second. The fastest person can run 100 metres in about 10 seconds.

Teacher: How far does light travel in just 1 second?

Teacher: It's more than 100 metres.

Teacher: It's more than 300 metres.

Teacher: It's about 300 million metres in each second!

Another way to approach this, for more mathematically able pupils:

Light travels 300 million metres (3 × 108 metre) in 1 second; or 300 thousand metres (3 × 105 metre) in 1 millisecond; or 30 kilometres (3 × 104 metre) in 0.1 millisecond.

In other words, a beam of light will travel from Newcastle to Sunderland (insert local reference) in one tenth of a thousandth of a second!

The main point here is that whilst the speed of light is extremely big it is also finite. Light does, therefore, take time to cover very large distances. For example, when you see the light from very remote stars, you are actually seeing light emitted from them a long time ago. You literally see them as they were in the distant past. Closer to home, you see the world 30 cm away from you as it was one nanosecond ago, not as it is now.

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