Astronomy

What Astronomy covers

Astronomy is examined in Unit AS 2, Waves, Photons and Astronomy, and drawn on again in Unit AS 3, Practical Techniques and Data Analysis. It covers the classical Doppler effect, the red shift parameter z, cosmological red shift and Doppler red shift, Hubble's law and the age of the universe.

The 5 subtopics

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

• The classical Doppler effectRecall questions on what the Doppler effect is and the ambulance siren as the everyday case, the wavefront explanation for an approaching and a receding source, the wavelength and frequency equations, worked calculations both ways, whether the emitted frequency itself changes, the null result for transverse motion, and the derivation of Δλ/λ = v/c.Open →
• The red shift parameter zRed shift and blue shift.Open →
• Cosmological red shift and Doppler red shiftRecall questions on what causes each kind of shift and the essential difference between them, where in the light's journey the stretching happens, which applies to a distant galaxy and which to an orbiting star, why Andromeda is blue-shifted, the inflating balloon analogy, and why the Earth is not at the centre.Open →
• Hubble's lawHubble's law, defining each symbol with its unit and value of the Hubble constant with its unit.Open →
• The age of the universeRecall questions on the estimate t ≈ 1/H₀ and how it follows from Hubble's law, worked calculations in seconds and in years, the assumption of a constant expansion rate and why it is not exactly right, the effect of a larger Hubble constant, why the oldest stars must be younger than the estimate, and the observable limit a finite age imposes.Open →

What you need to know

Answer each one before you reveal it.

Take Astronomy further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Red shift vs Blue shift

  Red shift: the received wavelengths are longer than the laboratory values and the separation is increasing. Blue shift: they are shorter and the separation is decreasing.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What is the Doppler effect?

   Model answer

   The Doppler effect is the change in the observed frequency and wavelength of a wave that occurs when the source and the observer are moving relative to one another.

2. 2

   Describe what is heard as an ambulance siren approaches, passes and drives away.

   Model answer

   The note is heard at a higher pitch than normal while the ambulance approaches, drops suddenly as it passes, and is heard at a lower pitch than normal as it recedes.

3. 3

   Explain in terms of wavefronts why an approaching source produces a shorter wavelength.

   Model answer

   The source moves forward between emitting one wavefront and the next, so each new wavefront starts from a point closer to the observer and the wavefronts ahead of the source are bunched together.

4. 4

   Explain in terms of wavefronts why a receding source produces a longer wavelength.

   Model answer

   The source moves away between emitting successive wavefronts, so each new wavefront starts further from the observer and the wavefronts behind the source are stretched apart.

5. 5

   State the classical equation for the wavelength received by a stationary observer from a receding source, defining each symbol.

   Model answer

   λ' = λ(c + v)/c, where λ' is the received wavelength in metres, λ is the wavelength emitted by the source at rest in metres, v is the speed of the source in m s⁻¹ and c is the speed of the wave in the medium in m s⁻¹.

6. 6

   State the classical equation for the wavelength received by a stationary observer from an approaching source.

   Model answer

   λ' = λ(c − v)/c, where λ' is the received wavelength, λ is the wavelength emitted by the source at rest, v is the speed of the approaching source and c is the wave speed, all in SI units.

What is inside Levo

Six ways to work the same material.

Astronomy revision questions

Is Astronomy examined at AS in CCEA Physics?

Yes. It sits in Unit AS 2, 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 Astronomy cover in CCEA AS-Level Physics?

5 subtopics: the classical doppler effect, the red shift parameter z, cosmological red shift and doppler red shift, hubble's law and the age of the universe. Each one has its own page with questions and model answers.

How is Astronomy organised?

By the CCEA 1210 specification, subtopic by subtopic, so it matches the order you are taught it in.