Compare matching galaxy and laboratory spectra
Use corresponding spectral features to recognise redshift in distant-galaxy light.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Define redshift.
- Read wavelength-labelled data.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why compare identified corresponding lines?
What you will learn
- Use corresponding spectral features to recognise redshift in distant-galaxy light.
- Check a pattern of matching lines instead of choosing unrelated peaks.
Match a spectral pattern
Light from distant galaxies generally shows features at longer wavelengths than corresponding radiation measured on Earth. Laboratory references identify line patterns, so several matching lines can be compared rather than judging the whole image by its colour.
A shifted pattern retains the identity relationships between features. In a simple common-fraction illustration, reference wavelengths 400, 500 and 600 nm appear at 408, 510 and 612 nm. The increases differ in nanometres, but each is 2% of its reference.
Check the comparison conditions
A shared wavelength calibration and reliable line identifications matter. A uniform instrument reading offset is not automatically a galaxy’s physical shift. Nor should a random observed peak be paired with any convenient laboratory value to create a desired result.
The syllabus’s distant-galaxy trend does not mean every nearby galaxy must recede. Local motions can give other outcomes. This lesson recognises the distant population evidence and does not require a detailed model for every individual galaxy.
Sources for this lesson
Pause and explain
What do longer matching wavelengths in a distant-galaxy spectrum indicate?
Worked example
Reference lines are 400, 500 and 600 nm; matching observed lines are 408, 510 and 612 nm. Describe the pattern.
Show the worked solution
- Match each pair by line identity.
- Find positive changes 8, 10 and 12 nm.
- Each change is 2% of its reference.
Answer All three matching features have longer observed wavelengths: a consistent redshift pattern.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Reference 450 nm and matching observation 459 nm are supplied. Describe the difference.
Give me a hint
Check identity and subtract in the same unit.
Compare my reasoning
- Use the stated matching pair.
- Calculate 459 − 450 = 9 nm.
- The observed wavelength increased.
A 9 nm redshift.
Look for these in your work
- Compare the corresponding feature.
- Name the longer-wavelength direction.
Reference 500 and 650 nm match observed 510 and 663 nm. Compare their fractional increases.
Give me a hint
Divide each increase by its own reference wavelength.
Compare my reasoning
- First: 10/500 = 0.02.
- Second: 13/650 = 0.02.
- The absolute increments differ but the fractions agree.
Both increase by 2%.
Look for these in your work
- Use each line’s own reference.
- Separate fractional and absolute shift.
All laboratory calibrations are reported 5 nm high, and galaxy lines show the same 5 nm offset. Is recession proved?
Give me a hint
A shared measurement error can mimic an absolute shift.
Compare my reasoning
- Identify the stated calibration problem.
- Correct or independently verify the wavelength scale.
- Reassess matching lines after correction.
No; the stated calibration offset must be resolved before attributing the shift to recession.
Look for these in your work
- Identify the confounder.
- Require a valid reference scale.
Common mistakes
- Use matching line identities.
- Absolute increments need not be identical.
- Nearby exceptions do not erase the distant trend.
What can you explain now?
What do longer matching wavelengths in a distant-galaxy spectrum indicate?
Compare with the explanation
A redshift relative to the Earth reference
The observed matching features have moved toward longer wavelengths.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Check a pattern of matching lines instead of choosing unrelated peaks.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.