Grade 12 Physical Sciences · Paper 1 (Physics)

Waves, Sound and Light Grade 12

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Question 1.6 · November 2025 · Paper 1 · National (DBE) · 2 marks

Various options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10) in the ANSWER BOOK, e.g. 1.11 E.

1.6 A star's spectrum, observed from Earth, is red shifted. Which ONE of the following statements is CORRECT?(2)

A The speed of light is increasing.
B The star is moving towards Earth.
C The frequency of each spectral line has increased.
D The wavelength of each spectral line has increased.

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Doppler Effect37 past paper questions

More Waves, Sound and Light Grade 12 questions and answers

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1. Question 6.1 · May/June 2025 · Paper 1 · National (DBE) · 3 marks
A bat emits sound waves with a frequency $f_{s}$ as it flies at a constant speed $v_{B}$ towards a vertical cliff. The waves reflect off the cliff with the same frequency that they strike the cliff, and with a wavelength of $λ_{L} = 0, 016$ m, as shown in the simplified diagram below.

Take the speed of sound in air to be $340$ m·s $^{- 1}$ .
6.1 NAME and STATE the phenomenon that results in a change in frequency of the detected sound waves.(3)
Show the memo (2 ticks)Memo
1. ✓
  NameDoppler effect
  1 mark · State and explain the Doppler effect
2. ✓
  StatementThe change in frequency (pitch) of the sound detected by a listener because the sound source and the listener have different velocities relative to the medium of sound propagation.
  OR
  A change in observed/detected frequency (pitch), as a result of the relative motion between a source and an observer (listener).
  2 marks · State and explain the Doppler effect
2. Question 1.6 · November 2024 · Paper 1 · National (DBE) · 2 marks
1.6 The absorption spectrum of an element surrounding a moving star is observed on Earth and found to be red shifted.

Which ONE of the following combinations is CORRECT for the movement of the star and the frequency of the observed light on Earth?(2)
1. A Movement of star: Away from Earth; Frequency of observed light on Earth: Decreased
2. B Movement of star: Towards Earth; Frequency of observed light on Earth: Decreased
3. C Movement of star: Away from Earth; Frequency of observed light on Earth: Increased
4. D Movement of star: Towards Earth; Frequency of observed light on Earth: Increased
Show the memo (1 tick)Memo
1. ✓
  Correct optionRed shift means the spectral lines are shifted towards longer wavelengths (the red end). A longer observed wavelength means a lower observed frequency. By the Doppler effect this happens when the source moves AWAY from the observer.
  
  C is the trap: the star moves away, but moving away gives a DECREASED frequency, not an increased one.
  
  Answer: A
  2 marks · State and explain the Doppler effect
3. Question 6.2 · November 2024 · Paper 1 · National (DBE) · 2 marks
A stationary listener, standing on the roadside, records the wavelength of the sound emitted by the siren of a police car travelling at a constant velocity.

In the wavelength-time graph below, NOT drawn to scale, $λ_{L}$ is the wavelength of the sound recorded by the listener and $λ_{S}$ is the wavelength of the sound emitted by the siren.

Take the speed of sound in air to be $343$ m·s $^{- 1}$ .
6.2 Is the car moving TOWARDS or AWAY FROM the listener? Give a reason for the answer.(2)
Show the memo (2 ticks)Memo
1. ✓
  DirectionAway from
  1 mark · State and explain the Doppler effect
2. ✓
  ReasonWavelength of sound detected/observed by the listener is longer than wavelength of sound emitted by the source.
  OR Frequency detected by the listener is lower than the frequency of the source.
  OR $λ_{L} > λ_{S}$ OR $f_{L} < f_{S}$
  1 mark · State and explain the Doppler effect
4. Question 1.6 · May/June 2024 · Paper 1 · National (DBE) · 2 marks
Various options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10).
1.6 The spectrum of helium emitted from a star moving away from Earth is compared to the spectrum of helium found on Earth.

Which ONE of the following statements is CORRECT?

The observed spectral lines from the moving star will have a ...(2)
1. A lower frequency and a longer wavelength.
2. B lower frequency and a shorter wavelength.
3. C higher frequency and a shorter wavelength.
4. D higher frequency and a longer wavelength.
Show the memo (1 tick)Memo
1. ✓
  Choose the correct optionA source moving away from the observer gives a lower observed frequency (Doppler effect). Since $c = f λ$ and the speed of light is constant, a lower frequency means a longer wavelength: the lines are shifted towards red (red shift).
  
  Answer: A
  2 marks · State and explain the Doppler effect
5. Question 6.1 · May/June 2024 · Paper 1 · National (DBE) · 2 marks
A stationary listener records the frequency of the sound emitted by the siren of a police car. When the car which is travelling at a constant velocity of $26$ m·s $^{- 1}$ , approaches the listener, the recorded frequency is $615$ Hz. The car passes the listener at time $t_{1}$ and then moves away from the listener. The recorded frequency now is $526$ Hz.

Ignore the effects of wind.
6.1 State the Doppler effect in words.(2)
Show the memo (1 tick)Memo
1. ✓
  Statement of the Doppler effectIt is the (apparent) change in frequency/pitch of the sound (detected by a listener) because the sound source and the listener have different velocities relative to the medium of sound propagation.
  
  OR
  
  An (apparent) change in (observed/detected) frequency/pitch as a result of the relative motion between a source and an observer (listener).
  2 marks · State and explain the Doppler effect

Waves, Sound and Light Grade 12 videos

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Doppler Effect grade 12: IntroductionKevinmathscience

The Doppler Effect Grade 12 Introduction and formulaMiss Martins Maths and Science

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