Redshift

What is Redshift?

Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies. It is measured by comparing the positions of identifiable spectral lines with their laboratory wavelengths.

This definition is part of the Space Physics chapter in Cambridge O Level Physics.

Redshift in context

Concepts related to Redshift

• Expanding Universe
• Big Bang Model
• Galaxy
• Light-Year

Common mistakes with Redshift

• Error Repair Common-Mistake Clinic Twenty-one errors that cost marks in Topic 6. Each one is stated as a student would write it, then corrected, then given an exam-safe replacement sentence and a retrieval check. The individual sections above develop several of these in more detail; this is the consolidated list to revise from. Scroll the table sideways on a narrow screen. The twenty-one Topic 6 misconceptions, with corrections and exam-safe replacements #The mistakeWhy it is wrong & the corrected modelExam-safe sentenceRetrieval check 1“Rotation and orbit mean the same thing.” Different axes, periods and consequences. Rotation spins the Earth about an axis inside itself in ~24 h; orbiting carries the Earth round the Sun in ~365 days. “The Earth rotates on its tilted axis in about 24 hours and separately orbits the Sun in about 365 days.” Which motion gives the day? Rotation. 2“Seasons occur because the Earth is much closer to the Sun in summer.” The orbit is approximately circular, so the distance barely changes; and the two hemispheres have opposite seasons at once. The axis is tilted. “The Earth's orbit is approximately circular; the Earth's axis is tilted.” Which observation rules out the distance explanation? Opposite seasons in the two hemispheres at the same time. 3“The Moon has no gravity.” Every mass has a gravitational field. The Moon's surface field strength is smaller than the Earth's because its mass is much smaller — not zero. “The Moon's surface gravitational field strength is smaller than the Earth's because the Moon has much less mass.” On the Moon, does an astronaut's mass or weight change? Weight only. 4“Gravity switches off in orbit.” If it did, the orbiting body would move in a straight line and leave. Apparent weightlessness happens because astronaut and spacecraft fall together. “The gravitational attraction still acts; it is what keeps the object in orbit.” What would the path become if gravity vanished? A straight line at constant speed. 5“A planet continues around the Sun because no force acts.” Circular motion requires a resultant force towards the centre. With no force there would be no curved path at all. “The gravitational attraction of the Sun provides the resultant force towards the centre.” Name the force that keeps a planet in orbit. Gravitational attraction of the Sun. 6“Centripetal force is an extra force alongside gravity.” “Centripetal” names a direction, not a separate force. For a planet, gravity is the inward resultant force. “The gravitational attraction of the Sun is the inward resultant force.” How many force arrows should a planet-orbit diagram carry? One inward arrow. 7“Planets further from the Sun orbit faster.” Inverted. Further out, the Sun's field is weaker and orbital speeds are lower: about 47 km/s at Mercury against about 5 km/s at Neptune. “As distance from the Sun increases, the Sun's field strength decreases and orbital speeds decrease.” Jupiter or Saturn — which is faster? Jupiter. 8“Orbital radius means orbital diameter.” Substituting a diameter for \(r\) doubles the circumference and doubles the answer. “\(r\) is measured from the centre of the central body; halve any diameter before substituting.” An orbit is 84 000 km across. What is \(r\)? \(4.2 \times 10^{7}\ \mathrm{m}\). 9“The Solar System and the Milky Way are the same.” The Solar System is one star and its orbiting bodies. The Milky Way is a galaxy of many billions of stars, containing the Solar System. “The Solar System is one star system inside the Milky Way galaxy.” Order by size: Earth, Milky Way, Solar System, Universe. Earth, Solar System, Milky Way, Universe. 10“The Sun is a planet.” The Sun releases energy by nuclear fusion and everything else orbits it. Planets do neither. “The Sun is a star — the only star in the Solar System.” What single test separates a star from a planet? A star releases energy by fusion. 11“Pluto is one of the eight planets.” Pluto is named in the syllabus as an example of a dwarf planet, a kind of minor planet. “Pluto is a dwarf planet, not one of the eight planets.” Name the eighth planet from the Sun. Neptune. 12“A light-year is a time.” The “year” says how long the light travelled; what is measured is how far it got. “One light-year is the distance travelled in a vacuum by light in one year.” Is a light-year a unit of distance or of time? Distance. 13“The Sun is the largest possible type of star.” The Sun is a star of medium size. Red giants and red supergiants are far larger. “The Sun is a star of medium size.” Give the syllabus word for the Sun's size. Medium. 14“Stars release energy by chemical burning.” Burning is chemical, needs oxygen, and releases far too little energy. Stars run on nuclear reactions. “The Sun releases energy by nuclear fusion of hydrogen into helium.” Rewrite “the Sun burns hydrogen”. “The Sun fuses hydrogen into helium.” 15“Fusion and fission are identical.” Opposites. Fusion joins small nuclei; fission splits a large one. Both release energy — that is all they share. “Fusion combines two smaller nuclei into a larger one, releasing energy.” Which process powers stars? Fusion. 16“No forces act on a stable star.” Two large opposing forces are in balance, not absent. “The inward force of gravitational attraction is balanced by an outward force due to the high temperature in the centre.” Name both forces in a stable star. Inward gravitational attraction; outward force from the high central temperature. 17“Every star becomes a supernova.” Only more massive stars do. A less massive star ends as a planetary nebula with a white dwarf at its centre. “The final stages depend on the mass of the star.” What is the final stage for a less massive star? A white dwarf at the centre of a planetary nebula. 18“Every star eventually becomes a black hole.” A black hole is one of two possible supernova remnants — the other is a neutron star — and only high-mass stars reach a supernova. “A supernova leaves a neutron star or a black hole.” Name both possible supernova remnants. Neutron star; black hole. 19“A planetary nebula contains newly formed planets.” The name is a historical accident. A planetary nebula is the shed outer layers of a red giant, with a white dwarf at its centre. “A planetary nebula is formed from the outer layers of a red giant and contains no planets.” What sits at the centre of a planetary nebula? A white dwarf. 20“Redshift means a galaxy simply looks red.” Redshift is a measured displacement of identified spectral lines towards longer wavelengths. A shifted violet line can still look blue. “The spectral lines appear at longer wavelengths than the same lines measured in a laboratory.” What must the observed spectrum be compared with? The laboratory reference spectrum. 21“Redshift alone proves every detail of the Big Bang” / “the Big Bang was an ordinary explosion at one place.” Redshift supports the model; it does not prove it. And the model describes space itself expanding everywhere, not material flying out from one location. “Redshift provides evidence that the Universe is expanding, which supports the Big Bang theory.” Replace “proves” with a safer verb. “Provides evidence for”.

Examiner tips on Redshift

• Answer Craft Examiner-Language Guidance What this section is and is not. The command-word meanings below are taken from the command-word list printed in the Cambridge O Level Physics 5054 syllabus for 2026–2028. Everything else on this page is Academiq Edu's own advice on phrasing. No examiner is quoted, no mark scheme is reproduced, and no claim is made about how often any idea has appeared in past examinations. Questions in this chapter are labelled exam-style because they were written for this resource; they are not past-paper questions. Command words that appear in Topic 6 questions Command words, their syllabus meanings, and what a Topic 6 answer should look like Command wordSyllabus meaningWhat that means here Stateexpress in clear termsOne short sentence, no explanation. “The gravitational attraction of the Sun.” Definegive precise meaningThe learned wording, complete. For orbital speed, name both the circumference and the period. Describestate the points of a topic / give characteristics and main featuresThe features in order, with no reasons required. Life-cycle sequences live here. Explainset out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidenceEvery sentence needs a because or a so. This is where the evidence chain belongs. Calculatework out from given facts, figures or informationEquation, conversion, substitution, answer, unit. Show all five. Determineestablish an answer using the information availableLike calculate, but you must first extract the numbers from a table or graph. Compareidentify/comment on similarities and/or differencesBoth items in every sentence: “Mercury orbits at 47 km/s whereas Neptune orbits at 5 km/s.” Suggestapply knowledge and understanding to situations where there are a range of valid responses in order to make proposals / put forward considerationsAn unfamiliar context. Name the physics you are applying, then apply it. Predictsuggest what may happen based on available informationUse the trend you have just established, and say which trend you used. Deduceconclude from available informationState the data you used, then the conclusion. Do not skip the first half. Justifysupport a case with evidence/argumentQuote the actual numbers or observations that support your claim. Commentgive an informed opinionA judgement plus the reason for it. Identifyname/select/recognisePick it out and name it. No description needed. Sketchmake a simple freehand drawing showing the key features, taking care over proportionsLabel both axes with quantity and unit, and get the shape right — a falling curve, not a falling straight line. Note The command word evaluate does not appear in the 5054 command-word list for this cycle. Where a question wants you to weigh evidence, it will use comment, justify, deduce or suggest instead. If you meet “evaluate” in a textbook exercise, treat it as comment plus justify. Phrasing that costs marks, and what to write instead Ten phrase swaps for Topic 6 Instead of…Write…Why “gravity pulls it round”“the gravitational attraction of the Sun acts towards the centre of the Sun”Names the source and the direction. “centripetal force keeps it in orbit”“the gravitational attraction of the Sun is the resultant force towards the centre”Avoids inventing a second force. “the speed changes” (of a circular orbit)“the speed is constant but the velocity changes because the direction changes”Speed and velocity are different quantities. “a light-year is how long light takes”“a light-year is the distance travelled in a vacuum by light in one year”A light-year is a distance. “the Sun burns”“the Sun fuses hydrogen into helium”Nuclear, not chemical. “the star explodes when it dies”“a red supergiant explodes as a supernova”Only the high-mass branch explodes. “it turns into a black hole”“a neutron star or a black hole is left at the centre”Both outcomes are in the syllabus. “the galaxy looks red”“the spectral lines are at longer wavelengths than the laboratory values”Redshift is a line shift, not a colour. “this proves the Big Bang”“this provides evidence for the Big Bang theory”Evidence supports models; it does not prove them. “it is hotter because it is closer”“Venus is further from the Sun than Mercury yet hotter, so distance is not the only factor; the atmosphere also matters”Quotes the data and names the competing factor. The three-second unit check. Before you write any numerical answer down, ask: is \(r\) in metres? Is \(T\) in seconds? Then the speed is in m/s. A period left in days is not a physics error, but it costs the same marks as one, and it is the easiest slip to make in this topic.

Questions students ask about Redshift