The Electromagnetic Spectrum
Light, Sound and Waves

A central role for the speed of light

Physics Narrative for 11-14 Supporting Physics Teaching

Light in physics

The speed of light plays a central role in astronomy and in physics.

According to Einstein's theory of relativity, no signal can travel faster than light. Furthermore, contrary to normal intuition, the theory of relativity tells us that light always travels at the same speed relative to an observer, no matter how that observer moves relative to the source of the light.

Thus, light emitted from a moving aeroplane does not travel with the speed of light plus the speed of the aeroplane, as recorded by an observer on Earth. It travels with the speed of light, no matter what the speed of the aeroplane. In a vacuum, light always travels at a speed of 3 × 108 metre/second, no matter how its speed is measured. Although this seems strange, it has been confirmed in many experiments. These experiments show that it is our common sense that is wrong in this case.

While these ideas are way beyond the scope of work appropriate for most 11–14 pupils, they are often very interested to hear about them, with the promise that they will find out more when they study advanced level physics.

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