Physics KS3 / GCSE: Measuring the speed of light

Professor Brian Cox explains how light travels so fast that people believed it was instantaneous.

A Danish astronomer, Ole Rømer, noticed that Jupiter’s moon, Io sometimes seemed to appear later or earlier than expected.

He concluded it was due to the relative position of the Earth and Jupiter at different times of the year.

If Jupiter is a long way away it takes longer for the light to travel to earth, and hence appears to be late.

He concluded that light takes time to travel and we now know this is at a constant speed of 299,792,458 m/s.

This clip is from the series Wonders of the Universe.

Teacher Notes

This clip could be used as part of a plenary exercise. Display all the key terms or words from the lesson on the board.

In pairs, students create questions, the answers of which will be the keywords or terms.

Divide the class into two teams. Each team puts forward a person with a fly swatter or similar object.

A series of questions about the topic are now asked (each pair taking it in turns to pose the question they compiled).

The team representatives swat the term they think is correct.

The team with the most correctly swatted answers wins.

Curriculum Notes

These clips will be relevant for teaching Physics at KS4 and GCSE in England, Wales and Northern Ireland and National 4/5 or Higher in Scotland.

The topics discussed will support OCR, Edexcel, AQA, WJEC GCSE in England and Wales, CCEA GCSE in Northern Ireland and SQA National 4/5 and Higher in Scotland.