Astrophysics question about a binary star system (AQA exam)

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The discussion centers on the interpretation of wavelength change in a binary star system as presented in an AQA exam mark scheme. The confusion arises from the mark scheme considering the amplitude of the wave rather than the peak-to-peak value when calculating the change in wavelength. The correct reference point for measuring this change is the unshifted wavelength of 656.28 nm, with a maximum shift of ± 0.24 nm. Participants seek clarification on why the mark scheme's approach is valid in this context. Understanding this distinction is crucial for accurately answering related astrophysics questions.
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Homework Statement
https://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/AQA-Old/Unit-5/June%202015%20QP%20-%20Unit%205-2A%20AQA%20Physics%20A-level.pdf
question 2
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Relevant Equations
change in wavelength
https://pmt.physicsandmathstutor.co...e 2015 MS - Unit 5-2A AQA Physics A-level.pdfmark scheme
question 2

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very simple question why is the change in wavlength not 656.05 - 656.52? instead.
The difference being in the mark scheme they have taken the greatest change in wavelength to be the amplitude of the wave not the peak to peak value. How does that make sense exactly.
 
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Because change in wavelength is measured relative to the unshifted wavelength of 656.28 nm. That is what we see when the velocity of the star relative to Earth is zero. The maximum shift is ± 0.24 nm.
 
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