Where does the continuous part of a stellar spectrum come from?

From the dense hot gas of the star's surface, which radiates at all wavelengths.

How does a line absorption spectrum arise in starlight?

Atoms in the star's cooler outer atmosphere absorb photons of the specific energies that raise their electrons between levels, removing those wavelengths from the continuous spectrum.

A body that absorbs all the radiation incident upon it, reflecting and transmitting none.

Why are stars treated as good approximations to black bodies?

They absorb essentially all radiation falling on them and their emitted spectrum closely matches the black body curve for their surface temperature.

WJEC AS-Level Physics 1420QS

Basic Physics

Topic overview Units and quantities Scalars, vectors and density Moments, centre of gravity and equilibrium Prefixes, derived units and unit conversion Adding coplanar vectors and the triangle of forces Resolving forces on an inclined plane Taking moments: beams, ladders and stability

Kinematics

Topic overview Describing motion Equations of motion and free fall Projectile motion Non-uniform acceleration and curved motion graphs Projectiles launched at an angle Measuring speed, acceleration and g in the laboratory

Dynamics

Topic overview Newton's second law and momentum Force and Newton's laws Impulse and the force–time graph Collisions and explosions in one dimension Free body diagrams and connected bodies Newton's third law pairs and everyday forces

Energy Concepts

Topic overview Work and power Conservation of energy and efficiency The work–energy relationship Gravitational potential energy and energy interchange Elastic potential energy and the force–extension graph Power, velocity and the efficiency of machines

Solids Under Stress

Topic overview Stress, strain and the Young modulus Stress–strain graphs and the behaviour of materials Measuring the Young modulus, and material properties Hooke's law and elastic strain energy Crystalline, amorphous and polymeric solids Dislocations, ductile and brittle fracture

Using Radiation to Investigate Stars

Topic overview Stellar spectra and black bodies Wien's law, Stefan's law and stellar properties The inverse square law and stellar distance Finding the radius and temperature of a star Multiwavelength astronomy

Particles and Nuclear Structure

Topic overview Quarks, leptons and hadrons The four interactions and conservation laws The first generation of quarks and leptons Antiparticles and annihilation Baryons, antibaryons and mesons Deducing quark compositions from charge Beta decay and quark flavour change Testing a reaction against the conservation rules

Conduction of Electricity

Topic overview Current, charge and the ampere Drift velocity and the equation I = nAvq The coulomb and the charge on an electron Conductors, insulators and other charge carriers The free-electron model and the speed of conduction

Resistance

Topic overview Calculating resistance from resistivity Potential difference, Ohm's law and power Resistivity, temperature and superconductivity The volt: energy transferred per unit charge Current–voltage characteristics of non-ohmic components Measuring resistance and resistivity in the laboratory The heating effect of a current and choosing a power equation Thermistors and how a semiconductor's resistance falls with temperature Conductance, conductivity and comparing materials

D.C. Circuits

Topic overview Kirchhoff's laws, and resistors in series and parallel Potential dividers, emf and internal resistance Cells, resistivity, power and energy Working out currents and potential differences in resistor networks Sensor circuits with thermistors and LDRs Internal resistance, power transfer and short-circuit current Ammeters, voltmeters and their effect on the circuit

The Nature of Waves

Topic overview What a wave is, and the wave equation Describing a wave: peaks, compressions and phase Polarisation and plane-polarised waves Displacement–time and displacement–position graphs Wavefronts and rays Phase difference between two points on a wave

Wave Properties

Topic overview Energy transfer and the wave equation Transverse and longitudinal waves, path difference and intensity Diffraction and superposition Two-source interference and the diffraction grating Stationary waves How much a wave spreads at a gap or an obstacle Coherence and the conditions for observable interference Young's double slit and λ = ay/D Measuring wavelength with a diffraction grating Measuring the speed of sound with stationary waves

Refraction of Light

Topic overview What refraction is and why it happens Refractive index and Snell's law Total internal reflection and optical fibres The wave model of refraction and n₁v₁ = n₂v₂ Snell's law at a boundary between two media Optical fibres: core, cladding and pulse broadening

Photons

Topic overview Photon energy and the electronvolt The photoelectric effect Stopping potential and the Planck constant The photoelectric effect and photon energies Line spectra and energy levels Demonstrating the photoelectric effect The electromagnetic spectrum and typical photon energies Electron diffraction and the de Broglie relationship Radiation pressure

Lasers

Topic overview Stimulated emission and the properties of laser light Population inversion, pumping and the laser cavity Why a two-level system cannot give a population inversion Three-level and four-level laser systems The semiconductor laser and its uses

Mathematical Requirements (Appendix C)

Topic overview Significant figures and arithmetic means Uncertainty and how uncertainties combine Algebraic symbols and proportionality Rearranging and solving equations Small angle approximations Tangents and instantaneous rates of change Areas under curves and numerical methods Reading a tangent on a motion graph Straight-line graphs and y = mx + c What a gradient and an intercept represent Linearising a relationship to find a constant Log plots of exponential decay Choosing what to plot to find a constant Logarithmic scales and orders of magnitude Working with base-ten logarithms The shapes of common graphs Inverse and root relationships on a graph Areas, volumes and Pythagoras Geometry applied to physical situations Substituting into physical equations Trigonometry and right-angled triangles Resolving a force into components Radians, arc length and force diagrams Random processes and translating between representations Mean values and periodic functions Symbols for approximation and comparison

How Science Works (Appendix D)

Topic overview Apparatus and taking measurements Working safely with radioactive sources Planning an investigation Analysing and evaluating evidence Uncertainty, repeatability and drawing a conclusion

Using Radiation to Investigate Stars

5 subtopics
15 things to know
AS Unit 1

What Using Radiation to Investigate Stars covers

Using Radiation to Investigate Stars is examined on AS Unit 1 of WJEC Physics, Motion, Energy and Matter. 67 recall questions across 5 subtopics.

Where it is assessed

AS Unit 1

Motion, Energy and Matter.

Check the WJEC specification ↗

The 5 subtopics

Each one has its own page. WJEC AS Physics students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

How is a star's surface temperature found from its spectrum?

Key term

What does Stefan's law say about the power a star radiates?

Key term

What two parts make up the spectrum observed from a star?

Key term

What can the absorption lines in a stellar spectrum tell an astronomer?

Key term

Define the luminosity of a star.

Key term

Define the intensity of the radiation received from a star.

Key term

Give Stefan's law for the total power radiated by a star, defining every symbol.

Key term

State the value and unit of the Stefan constant.

Process

Why does observing at different wavelengths reveal different processes?

Key term

What does an infrared observation reveal that a visible one cannot?

Key term

Define a black body.

Key term

State Wien's displacement law in words.

Key term

Give the inverse square law relating intensity to luminosity and distance, and define every symbol.

Key term

Rearrange Stefan's law to give the radius of a star.

Key term

What does a radio observation reveal?

Take Using Radiation to Investigate Stars further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

A continuous emission spectrum vs A line absorption spectrum
A continuous emission spectrum: an unbroken band of all wavelengths from a hot dense gas. A line absorption spectrum: dark lines where a cooler gas has removed particular wavelengths.
Luminosity vs Intensity
Luminosity: the total power a star radiates in all directions. Intensity: the power received per unit area at a given distance.
Wien's displacement law vs Stefan's law
Wien's displacement law: the rule that peak wavelength is inversely proportional to absolute temperature. Stefan's law: the rule that radiated power depends on area and the fourth power of absolute temperature.

Questions from this topic

Each one with the model answer underneath.

1
What two parts make up the spectrum observed from a star?
Model answer
A continuous emission spectrum from the dense gas of the star's surface, and a line absorption spectrum produced by the cooler gas of its atmosphere.
2
Where does the continuous part of a stellar spectrum come from?
Model answer
From the dense hot gas of the star's surface, which radiates at all wavelengths.
3
How does a line absorption spectrum arise in starlight?
Model answer
Atoms in the star's cooler outer atmosphere absorb photons of the specific energies that raise their electrons between levels, removing those wavelengths from the continuous spectrum.
4
What can the absorption lines in a stellar spectrum tell an astronomer?
Model answer
Which elements are present in the star's atmosphere, because each element absorbs at a characteristic set of wavelengths.
5
Define a black body.
Model answer
A body that absorbs all the radiation incident upon it, reflecting and transmitting none.
6
Why are stars treated as good approximations to black bodies?
Model answer
They absorb essentially all radiation falling on them and their emitted spectrum closely matches the black body curve for their surface temperature.

See the rest and revise

What is inside Levo

Six ways to work the same material.

1Closed book

2Open book

3Closed book

The 3-step method, run for you

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Basic Physics

Kinematics

Dynamics

Energy Concepts

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

R is the .

radiusdiametercircumference

Blank Blitz makes facts automatic

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AF is force.

BB is magnetic flux density.

CI is current.

Build It gets the exam shape right

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

A baryon means a particle built from three quarks.

TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Using Radiation to Investigate Stars revision questions

Is Using Radiation to Investigate Stars examined at AS in WJEC Physics?: Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Using Radiation to Investigate Stars cover in WJEC AS-Level Physics?: 5 subtopics: stellar spectra and black bodies, wien's law, stefan's law and stellar properties, the inverse square law and stellar distance, finding the radius and temperature of a star and multiwavelength astronomy. Each one has its own page with questions and model answers.
How is Using Radiation to Investigate Stars organised?: By the WJEC 1420QS specification, subtopic by subtopic, so it matches the order you are taught it in.

Every question in Using Radiation to Investigate Stars, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Start revising, £5 a month See how the method works

Or £46 a year, about £3.83 a month. Cancel anytime.