Describe redshift as increased observed wavelength
Describe redshift by comparing longer observed wavelengths with their emitted reference wavelengths.
Section 6.2.3 · The Universe
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use wavelength as peak-to-peak separation.
- Recall the electromagnetic spectrum ordering.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is wavelength measured in a repeating wave?
What you will learn
- Describe redshift by comparing longer observed wavelengths with their emitted reference wavelengths.
- Distinguish redshift from red colour, dimming and a change in the source’s chemical identity.
Compare the same radiation feature
Redshift is an increase in the observed wavelength of electromagnetic radiation emitted by a receding star or galaxy, compared with the corresponding emitted reference. In visible light, longer wavelengths lie toward the red end, which motivates the name. Radiation need not become visibly red for a measurable redshift.
Use matching features: for example, compare a particular spectral line with the laboratory wavelength of that same transition. Comparing two unrelated lines can produce a wavelength difference even with no source recession, so line identification matters.
Separate observable changes
Dimming is a change in measured brightness, while redshift concerns wavelength. A red-coloured star can have a different surface temperature without the colour alone establishing a recession measurement. The spectral comparison gives stronger evidence than a red tint in an image.
For electromagnetic radiation measured under the same propagation conditions, longer wavelength corresponds to lower frequency. This is a wave relationship, not a claim that the emitting element has turned into another element. The lesson’s wavelength examples are supplied values, not telescope observations collected by this website.
Pause and explain
Which observation is a redshift?
Worked example
A matched reference line is 500 nm and the observed line is 515 nm. Describe its shift.
Show the worked solution
- Compare the same identified line.
- Calculate observed minus reference: 15 nm.
- The increase means redshift.
Answer Redshift: the observed wavelength is 15 nm longer.
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.
Compare reference 600 nm with observed 612 nm. State the shift and direction.
Give me a hint
Subtract the reference wavelength from the matching observed wavelength.
Compare my reasoning
- Use the same spectral feature.
- Calculate 612 − 600 = 12 nm.
- An increase corresponds to redshift.
12 nm longer; redshift.
Look for these in your work
- Keep feature identity.
- State increased wavelength.
A matched line changes from 600 nm to 594 nm. Is that a redshift?
Give me a hint
An observed decrease has the opposite sign from the definition.
Compare my reasoning
- Calculate 594 − 600 = −6 nm.
- The wavelength is shorter.
- Do not label a decrease as redshift.
No; it is a 6 nm decrease, conventionally a blueshift.
Look for these in your work
- Give the sign.
- Apply the wavelength definition.
An image is red-tinted but no spectral reference is supplied. Can it establish a recession redshift?
Give me a hint
A colour appearance can have causes other than a measured line shift.
Compare my reasoning
- Colour can depend on surface temperature or image processing.
- Redshift needs corresponding wavelength comparison.
- Request an identified spectral reference and observation.
No; the tint alone does not establish a measured redshift.
Look for these in your work
- Distinguish colour from spectral shift.
- State needed evidence.
Common mistakes
- Redshift is not simply red appearance.
- Dimming and wavelength change differ.
- Compare matching spectral features.
What can you explain now?
Which observation is a redshift?
Compare with the explanation
A matched line moves from 500 nm to 515 nm
The observed wavelength of the same feature has increased.
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Make it stick
Distinguish redshift from red colour, dimming and a change in the source’s chemical identity.
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.