Physics • Year 12 • Module 7 • Lesson 9
Spectroscopy and Astronomical Applications
Lock in the core vocabulary, Kirchhoff’s three laws, and the Doppler shift formula before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: spectroscopy, continuous spectrum, emission spectrum, absorption spectrum, Fraunhofer lines, redshift, blueshift, Doppler effect, redshift parameter z, radial velocity. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The study of the interaction between matter and electromagnetic radiation as a function of wavelength. | |
| 1.2 | A spectrum showing all wavelengths of visible light without gaps; produced by a hot solid, liquid or dense gas. | |
| 1.3 | A spectrum of bright lines at specific wavelengths produced when excited atoms emit photons as electrons drop to lower energy levels. | |
| 1.4 | A continuous spectrum with dark lines at specific wavelengths where photons have been absorbed by a cooler gas in front of the source. | |
| 1.5 | Dark absorption lines in the Sun’s spectrum caused by elements in the cooler outer solar atmosphere absorbing light from the hotter interior. | |
| 1.6 | An increase in the observed wavelength of light from a source that is moving away from the observer. | |
| 1.7 | A decrease in the observed wavelength of light from a source that is moving toward the observer. | |
| 1.8 | The change in observed frequency or wavelength of a wave when the source and observer are moving relative to each other. | |
| 1.9 | The dimensionless quantity defined as Δλ/λ0, giving the fractional shift in wavelength of a spectral line. | |
| 1.10 | The component of a star’s velocity directed along the line of sight toward or away from the observer; measurable via Doppler shifts. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 A hot thin gas produces a continuous spectrum because all wavelengths of light are emitted uniformly. T / F
2.2 In an absorption spectrum, dark lines appear at exactly the same wavelengths as the bright lines in the emission spectrum of the same element. T / F
2.3 When a light source moves away from an observer, the observed wavelength decreases (blueshift). T / F
2.4 The Doppler formula Δλ/λ0 = v/c is valid for source speeds much less than the speed of light (v ≪ c). T / F
2.5 Spectroscopy can determine the chemical composition of a star because every element produces a unique and characteristic pattern of spectral lines. T / F
2.6 Edwin Hubble’s observations showed that distant galaxies are blueshifted, indicating the universe is contracting. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
absorption · blueshift · composition · continuous · emission · Fraunhofer · radial · redshift
A hot solid or dense gas produces a ___________ spectrum across all visible wavelengths, whereas a hot thin gas produces an ___________ spectrum consisting of bright lines at wavelengths unique to each element. A cool thin gas placed in front of a continuous source creates an ___________ spectrum, where dark lines called ___________ lines appear at those same characteristic wavelengths. By matching these lines to known laboratory spectra, astronomers can determine the chemical ___________ of distant stars. When a star moves away from Earth its spectral lines shift to longer wavelengths — a ___________. Conversely, a star approaching Earth shows a ___________. The magnitude of the shift depends on the ___________ velocity of the source according to Δλ/λ0 = v/c.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the quantum mechanical explanation for why an emission spectrum contains discrete bright lines rather than a continuous spread of colours?
4.2 What is the function of spectral lines as “atomic fingerprints” in astronomical observations?
4.3 Explain why the dark lines in the Sun’s absorption spectrum are produced in the solar atmosphere rather than in its core.
4.4 A star exhibits periodic Doppler shifts that alternate from redshift to blueshift and back again with a period of 4 years. What does this most likely indicate?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “produces”, “is evidence of”, “is measured by”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: emission spectrum · absorption spectrum · electron energy levels · Doppler shift · radial velocity · chemical composition.
6. Kirchhoff’s three laws — complete the table
Complete the table for each of Kirchhoff’s three laws of spectroscopy. 9 marks (1 per cell)
| Law # | Source condition | Spectrum type produced | Everyday or astronomical example |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 |
Q1 — Term–definition match
1.1 spectroscopy • 1.2 continuous spectrum • 1.3 emission spectrum • 1.4 absorption spectrum • 1.5 Fraunhofer lines • 1.6 redshift • 1.7 blueshift • 1.8 Doppler effect • 1.9 redshift parameter z • 1.10 radial velocity.
Q2 — True / false with correction
2.1 False. A hot thin gas produces an emission spectrum (bright lines). A hot solid, liquid, or dense gas produces a continuous spectrum.
2.2 True.
2.3 False. When a light source moves away, the observed wavelength increases (redshift), not decreases. Blueshift occurs when the source moves toward the observer.
2.4 True.
2.5 True.
2.6 False. Distant galaxies are redshifted, indicating the universe is expanding. This was Hubble’s discovery, not a contraction.
Q3 — Cloze paragraph
In order: continuous / emission / absorption / Fraunhofer / composition / redshift / blueshift / radial.
Q4.1 — Quantum explanation for discrete spectral lines
Atoms have discrete (quantised) electron energy levels. An emission line is produced only when an electron drops from a higher energy level to a lower one, emitting a photon with energy E = hf equal to the energy difference between the two levels. Because the energy levels are fixed and unique to each element, only specific wavelengths are emitted.
Q4.2 — Function of spectral lines as “atomic fingerprints”
Each element has a unique pattern of energy levels and therefore a unique set of spectral lines. By matching the pattern of lines in a star’s spectrum to laboratory spectra of known elements, astronomers can identify which elements are present in the star’s atmosphere, even when the star is billions of light-years away.
Q4.3 — Why Fraunhofer lines form in the solar atmosphere
The Sun’s hot interior acts as a source of continuous (blackbody) radiation. The cooler outer atmosphere (chromosphere) contains atoms that absorb specific wavelengths from this continuous background, exciting their electrons to higher levels. This produces dark absorption lines at those wavelengths. The core itself is too dense and hot to produce isolated line absorption; the thin outer atmosphere provides the right conditions (Kirchhoff’s third law).
Q4.4 — Periodic Doppler shifts indicating orbital motion
Periodic, symmetric Doppler shifts most likely indicate that the star is in a binary system — it is orbiting an unseen companion (which may be another star, a black hole, or an exoplanet-mass body) about a common centre of mass. The 4-year period is the orbital period of the system.
Q5 — Sample concept map
Accept any valid labelled arrows, for example:
- electron energy levels — determine wavelengths of → emission spectrum
- electron energy levels — determine wavelengths of → absorption spectrum
- emission spectrum — reveals → chemical composition
- absorption spectrum — reveals → chemical composition
- Doppler shift — measures → radial velocity
- absorption spectrum — shifts indicate → Doppler shift
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).
Q6 — Kirchhoff’s three laws
Law 1: Source = hot solid, liquid or dense gas — Spectrum = continuous (all wavelengths, no gaps) — Example: the Sun’s interior / a filament globe / molten iron at a steelworks.
Law 2: Source = hot thin gas — Spectrum = emission (bright lines at specific wavelengths) — Example: a neon sign / a hydrogen discharge tube / a nebula.
Law 3: Source = continuous background + cool thin gas in the line of sight — Spectrum = absorption (continuous with dark lines at same wavelengths as the emission lines of the cool gas) — Example: the Sun’s Fraunhofer lines / absorption lines in the spectrum of a distant star passing through a cool gas cloud.