Light from hydrogen in a laboratory has a dark line in its absorption spectrum at a wavelength of 656 nm656\ \text{nm}656 nm.
An astronomer measures the wavelength of the same line in the light from four galaxies, A, B, C and D.
The Table shows her results.
Table
| Galaxy | Wavelength of the line in nm\text{nm}nm |
|---|---|
| A | 663 |
| B | 650 |
| C | 689 |
| D | 676 |
Identify the galaxy that is moving towards the Earth.
Give a reason for your answer.
Calculate the change in wavelength of the line for galaxy C.
Put galaxies A, C and D in order of their distance from the Earth, starting with the nearest.
Explain your answer.
A student says:
'Galaxy B is moving towards us, so the Universe cannot be expanding.'
Explain why the student is wrong.
94 exam-style questions on Edexcel GCSE Physics 7.2 The Universe and red-shift, covering 7.2.1 Big Bang and Steady State theories, 7.2.2 Evidence for the Big Bang, 7.2.3 The Doppler effect and red-shift, 7.2.4 Red-shift as evidence for an expanding Universe, and 7.2.5 The CMB and the accepted model. Each one has a worked solution and a mark scheme showing where the marks go.