A LevelPhysicsAQARevision NotesTurning Points in PhysicsSpecial RelativityThe Invariance of the Speed of Light

The Invariance of the Speed of Light (AQA A Level Physics): Revision Note

Exam code: 7408

Dan Mitchell-Garnett

Written by: Dan Mitchell-Garnett

Reviewed by: Tim

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The invariance of the speed of light

The speed of light

  • In a vacuum, light travels at a speed of 3.0 × 108 m s-1, which is given the symbol c

  • When travelling through different materials, the speed of light may reduce but it can never exceed c

What does invariance mean?

  • The invariance of light means that when in a vacuum, the speed of light is always 3.0 × 108 m s-1 for every observer

    • Light still travels at this speed even when emitted from a moving object

A runner holding a torch and a stationary observer both measure the torch's light travelling at the same speed, c.
Light does not travel at the speed of light c plus the speed of the person running, to all observers it just travels at the speed of light c

  • This concept only applies to light, as it is the only thing capable of travelling at the speed of light

    • The invariance of the speed of light has some very surprising consequences, which will be explored in the following revision notes...

Examiner Tips and Tricks

The invariance of the speed of light may feel counter-intuitive at first, but remember that nothing can travel faster than the speed of light, so if you ever end up with an answer for velocity bigger than 3.0 × 108 m s-1, something has gone wrong.

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Dan Mitchell-Garnett

Author: Dan Mitchell-Garnett

Expertise: Physics Content Creator

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to Save My Exams. Here, he carries on his passion for writing challenging physics questions and helping young people learn to love physics.

Tim

Reviewer: Tim

Expertise: Content Creator

Timothy graduated with a first class degree in Mathematics and Physics from the University of Warwick. After working as a postgraduate researcher, Timothy has worked as a content creator for various online revision platforms, creating physics resources for a range of levels and exam boards.