THE DOPPLER EFFECT
4 marks · NormalThe diagram shows lines on the spectrum of light from two galaxies, Q and R, at different observed wavelengths.
State which galaxy, Q or R, is moving away from Earth faster. Justify your answer using the spectrum diagram.
The normal wavelength of a particular line in a laboratory is 590 nm. For Galaxy Q, this line is observed at 594 nm. For Galaxy R, it is observed at 600 nm. Calculate the change in wavelength () for both galaxies.
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(a) Galaxy R is moving away faster, because its observed line is at a longer wavelength (greater red-shift) than Galaxy Q.
(b) Q: Δλ = 594 nm - 590 nm = 4 nm. R: Δλ = 600 nm - 590 nm = 10 nm.
Common mistakes: Mixing up which change is greater
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Marking points
B1 (a) · 1 mark · Identifies R as moving away faster.
B1 (a) · 1 mark · Correct justification: greater red-shift/shift to longer wavelength.
M1 (b) · 1 mark · Correct calculation for Q: Δλ = 594 nm - 590 nm = 4 nm.
M1 (b) · 1 mark · Correct calculation for R: Δλ = 600 nm - 590 nm = 10 nm.
Worked solution
a) Galaxy R is receding faster because its spectrum line is observed at a longer wavelength (600 nm) compared to Q (594 nm), giving R a greater red-shift.
b) Q: R: