Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
7.12P Describe the red-shift in light received from galaxies at different…
Full specification wording
Topic 7 – Astronomy: 7.12P Describe the red-shift in light received from galaxies at different distances away from the Earth
Light from a galaxy contains characteristic lines at known wavelengths.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 11 of 18
Study notes
From the checked teaching notes for this specification point.
The idea
Light from a galaxy contains characteristic lines at known wavelengths. When a galaxy is receding, its light is shifted towards the red end of the spectrum, so the observed wavelengths are longer than the emitted ones. This is red-shift. Observations show that the more distant a galaxy is, the greater its red-shift, which means the more distant galaxies are receding faster. This evidence supports the idea that the Universe is expanding. A nearby galaxy shows only a small red-shift, while a very distant galaxy shows a large one. The pattern of increasing red-shift with distance is the key observation to describe.
What earns the marks
- States that light from a receding galaxy is shifted towards the red end of the spectrum, giving longer observed wavelengths.
- Describes that the red-shift is greater for galaxies that are further away from the Earth.
- Links the increasing red-shift with distance to the conclusion that more distant galaxies recede faster.
- Connects the observed red-shift of galaxies to evidence that the Universe is expanding.
Common mistakes and how to put them right
Claims that red-shift means a galaxy is approaching.
Put it right by linking red-shift to recession and longer wavelengths.
Says the red-shift is the same for all galaxies.
Put it right: Describe how it increases with distance.
Confuses red-shift with the galaxy actually turning red.
Put it right: Explain it as a shift of spectral lines to longer wavelengths.
Exam tips
- State the direction of the shift (towards the red end) and what that means for wavelength.
- Compare near and far galaxies explicitly to show the distance relationship.
- Finish by stating what the pattern tells us about the Universe, namely that it is expanding.
Try the lesson
The start of the Topic 7 – Astronomy: 7.12P Describe the red-shift in light received from galaxies at different distances away from the Earth lesson, made from checked teaching notes for this specification point.
Have a go first
Light from a distant galaxy shows its spectral lines shifted towards the red end of the spectrum. What does this red-shift tell us about the galaxy's motion relative to Earth?
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