Video · Teachers Media
Our Universe and the Big Bang - Classroom Demonstration of Redshift
A specific classroom demonstration of redshift — the phenomenon that gave us direct evidence for an expanding universe. Redshift is one of those ideas that lives entirely on its evidence, and a physical demonstration turns an abstract equation into something pupils can point at.
About this video
Redshift is what happens when a wave source is moving away from the observer: the wavelengths get stretched, so light shifts toward the red end of the spectrum and sound drops in pitch. For distant galaxies, redshift is the evidence that space itself is expanding — the wavelength of the light we receive has been stretched by the expansion during its journey to us.
For pupils, the cleanest way in is the acoustic parallel — the Doppler effect. A police siren changing pitch as the car passes is redshift they've heard. A tuned fork on a string, whirled around a head, does the same demonstration safely in a classroom. Once pupils have the sound analogy, moving to the light case (and to cosmological redshift, which is subtly different from ordinary Doppler shift) is a smaller step.
The subtle teaching point is that cosmological redshift is not a Doppler shift caused by galaxies flying away through space — it's a stretching of the light's wavelength as space itself expands underneath it during transit. That distinction matters for higher-attaining KS4 and A-level pupils; for lower KS4 the Doppler analogy is enough.
A working sequence for teaching redshift
- Establish the Doppler effect for sound (siren, whirled tuning fork, or online simulation). Ask pupils to explain the pitch change in terms of wavelength.
- Transfer the idea to light. If a light source moves away, its wavelengths stretch — red end of the spectrum.
- Show a real redshift spectrum: absorption lines of a distant galaxy shifted compared to a nearby one. Ask pupils what this tells us.
- Introduce the cosmological picture: the redshift of the most distant galaxies is not a simple Doppler shift but the stretching of light by expanding space.
Discussion prompts
- What's the difference between the Doppler shift you get from a moving car and the cosmological redshift you get from a distant galaxy?
- How do you help pupils see that redshift is evidence, not just a fact?
- Which of your demonstrations most reliably shifts a misconception, and what makes them work?
Try this in your classroom
- Whirl a small speaker or a tuning fork on a string (safely, in an open space). Ask pupils to describe the pitch change and explain it.
- Project a real galaxy spectrum showing shifted absorption lines. Get pupils to point at the shift and explain what it means.
- Ask pupils to design an experiment that would show whether a distant object is moving toward or away from us. What would count as evidence?