Astrophysics question about a binary star system (AQA exam)

[bonbon22]

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
image below

Relevant Equations

change in wavelength

https://pmt.physicsandmathstutor.co...e 2015 MS - Unit 5-2A AQA Physics A-level.pdfmark scheme
question 2

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.

 

[mjc123]

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.