Practise 37 exam-style questions on Red-shift for AQA GCSE Physics. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
37 questions
1–6 marks each
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20 questions, marked as you go
Sample questions with model answers
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Question 1
2 marks
Astronomers have measured the red-shift of light from galaxies at many different distances from the Earth.
Describe how the red-shift and the speed of a galaxy depend on its distance from the Earth.
Model answer
The further away a galaxy is, the bigger the observed red-shift of its light, which means the faster it is moving away from us.
Explanation
A bigger red-shift means a faster recession speed. The pattern that more distant galaxies recede faster is what you would expect if the whole universe is expanding.
Mark scheme
1 mark
the further away a galaxy is, the bigger its red-shift
1 mark
the further away a galaxy is, the faster it is moving away
allow speed is (directly) proportional to distance
Question 2
2 marks
Astronomers measure red-shift by looking at the dark absorption lines in a galaxy's spectrum, rather than at the overall colour of the galaxy.
Suggest why.
Model answer
Each element absorbs light at fixed, known wavelengths, so the pattern of lines can be recognised. By comparing the position of each line with the same line in the Sun's or a laboratory spectrum, the shift in wavelength can be measured accurately. The overall colour of a galaxy depends on other things, such as the temperature of its stars, so it would not show the shift clearly.
Explanation
Spectral lines act like a barcode with known positions. Measuring how far the barcode has moved gives a precise value for the red-shift.
Mark scheme
1 mark
each element produces lines at fixed / known wavelengths (so the pattern can be recognised)
1 mark
so the shift of each line can be measured accurately by comparing with the same lines from the Sun / a laboratory source
allow the overall colour also depends on other things such as temperature
Question 3
3 marks
Light observed from a distant galaxy has a wavelength of 520 nm.
The speed of light is 3.0 × 10⁸ m/s.
Calculate the frequency of the observed light. Give your answer to 2 significant figures.
Model answer
wavelength = 520 nm = 520 × 10⁻⁹ m = 5.2 × 10⁻⁷ m wave speed = frequency × wavelength 3.0 × 10⁸ = f × 5.2 × 10⁻⁷ f = 3.0 × 10⁸ ÷ 5.2 × 10⁻⁷ = 5.77 × 10¹⁴ Hz f = 5.8 × 10¹⁴ Hz (2 s.f.)
Explanation
Nano means 10⁻⁹, so convert nanometres to metres before using v = f λ. If the galaxy's light has been red-shifted, this observed frequency is lower than the frequency that was emitted.
Mark scheme
1 mark
520 nm = 5.2 × 10⁻⁷ m
an answer of 5.8 × 10¹⁴ (Hz) scores 3 marks
1 mark
3.0 × 10⁸ = f × 5.2 × 10⁻⁷
allow f = 3.0 × 10⁸ ÷ 5.2 × 10⁻⁷; allow ecf from an incorrect conversion
1 mark
f = 5.8 × 10¹⁴ (Hz)
answer to 2 significant figures; allow ecf
Question 4
4 marks
A newspaper article says: "The Big Bang was a huge explosion at one point in space. Ever since, the galaxies have been flying away from the centre of the explosion."
Evaluate the newspaper's description using what scientists know about red-shift and the expanding universe.
Model answer
The article is right that the universe began from a very small region and that galaxies are moving apart, as shown by red-shift. However, the Big Bang was not an explosion of matter into empty space that already existed: space itself has been expanding ever since, carrying the galaxies apart. There is no centre. Red-shift is seen in every direction, with more distant galaxies receding faster, and an observer in any galaxy would see the same thing. So the description is partly correct but misleading.
Explanation
An evaluation should say what is right and what is wrong, with reasons, and reach a judgement. The key correction is that the expansion is of space itself and has no centre.
Mark scheme
1 mark
correct: the universe began from a very small region and galaxies are moving apart (shown by red-shift)
max 4
1 mark
but the Big Bang was not an explosion of matter into empty space that already existed
1 mark
space itself is expanding, carrying the galaxies apart
1 mark
there is no centre: from any galaxy, other galaxies appear to recede in every direction
1 mark
red-shift is seen in all directions, with more distant galaxies receding faster
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