C.5.4—Spectral line shifts
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Explain Spectral-Line Shifts
Use the line pattern as a fingerprint
Elements produce characteristic sets of spectral lines. Compare the same line pattern measured in a laboratory with the pattern observed from a star or galaxy. If every characteristic line is displaced by the same fractional amount while the pattern remains recognisable, the displacement is evidence of relative motion along the line of sight.
Interpret redshift and blueshift
A shift toward longer wavelengths and lower frequencies is a redshift, indicating recession along the line of sight. A shift toward shorter wavelengths and higher frequencies is a blueshift, indicating approach. The line pattern itself identifies the element; the displacement carries the motion information.
State exactly what the shift reveals
A Doppler spectral shift gives the component of relative motion along the observer’s line of sight. Redshift indicates that separation is increasing; blueshift indicates that it is decreasing. The shift alone does not measure motion across the line of sight, and it does not imply that the emitting element or the speed of light has changed.
Common trap
Do not say that a redshift means the element has changed. The spectral identity remains in the line pattern; the wavelengths have shifted because of relative motion.
C.5.4 Exam Analysis
Assessment in practice
1–2 marksHow it is assessed
Questions interpret a shifted line spectrum or outline why redshifts from distant galaxies support an expanding universe.
Command terms
Determine / Outline
What earns marks
Match observed and laboratory line patterns, identify redshift or blueshift, state the corresponding recession or approach, and connect systematic distant-galaxy redshift to cosmic expansion.
Watch for
Confusing redshift with a change in light speed, or treating one shifted line as sufficient evidence without matching the spectral pattern.
Representative question
Question 1
[Maximum number: 2]
The diagram below shows the spectrum of the stars as observed from Earth. The spectrum shows one line from star A and one line from star B, when the stars are in the position shown in the diagram (b).
On the spectrum draw lines to show the approximate positions of these spectral lines after the stars have completed one quarter of a revolution.
line from star B line from star A
increasing wavelength
Award [1] for each correct line.
The shifted lines are light grey in the diagram above. Ignore magnitude of shift. Award [0] if more than two lines are drawn unless it is clear which lines are to be marked.
Retrieve the Core C.5 Doppler Effect Model
C.5 Doppler effect is secure when you can connect relative motion to the observed wave.
- Approach raises observed frequency; recession lowers it
- A moving source compresses or spreads wavefront spacing
- A moving observer changes encounter rate, not medium wavelength
- For light at low relative speed, Δf/f=Δλ/λ≈v/c
- Spectral-line shifts reveal motion of stars and galaxies