The Physics Question Bank

Doppler Effect and Red Shift (A level)

✏️ Paper first! Work out every question on paper before you open the answer. Write down every step – the equation, the numbers with units, the rearranging and the answer with its unit. In the exam, if your final answer is wrong you can still get marks for correct working, but only if the examiner can see it.

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For A level physics. These topics are in the AQA Astrophysics option (and in OCR A and Edexcel). Watch wavefronts bunch up in front of a moving source, slide a galaxy’s spectrum lines towards red or blue, then practise red shift, Hubble’s law, binary stars, radar and the evidence for the Big Bang.

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Worked solution

    Key facts

    A moving source squashes the waves in front (shorter wavelength, higher frequency or pitch) and stretches the waves behind (longer wavelength, lower frequency). The wave speed does not change.

    z = Δf/f = −Δλ/λ = v/c   (v much less than c)

    Use the sizes: v = c|Δλ|/λ. A red shift (longer λ) means moving away; a blue shift means moving towards us. c = 3.00×108 m s−1 = 3.00×105 km s−1.

    v = H₀d     age of universe ≈ 1/H₀

    H₀ = 65 km s−1 Mpc−1 (data booklet). 1 pc = 3.08×1016 m, so 1 Mpc = 3.08×1022 m. 1 year = 3.15×107 s. Convert H₀ to s−1 before finding 1/H₀.

    Evidence for the Big Bang: galaxies’ red shifts increase with distance, the cosmic microwave background radiation, and the relative abundance of hydrogen and helium.

    Answers within 2% are marked correct. Answers read from a diagram or graph allow for the reading accuracy (up to 4%), and wavelengths given to 0.1 nm allow ±0.15 nm.

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