MCQs / Physics / Wave Intensity and the Doppler Effect

Wave Intensity and the Doppler Effect MCQs — Physics

60 multiple-choice questions on Wave Intensity and the Doppler Effect from Physics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

1. A source of sound is moving to the right. An observer on the left hears:

  1. a higher frequency
  2. the same frequency
  3. no sound
  4. a lower frequency

Answer: D) a lower frequency

Explanation: Source receding. SLO P-11-D-02 · Understanding

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2. The Doppler effect does NOT change:

  1. the frequency emitted by the source
  2. the wavelength heard ahead
  3. the pitch heard
  4. the frequency heard

Answer: A) the frequency emitted by the source

Explanation: Only the observed frequency changes. SLO P-11-D-02 · Understanding

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3. Bats use echoes (and Doppler shifts) to:

  1. hear music
  2. measure temperature
  3. see colours
  4. locate and track moving insects

Answer: D) locate and track moving insects

Explanation: Echolocation. SLO P-11-D-04 · Understanding

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4. A wave delivers a power of 60 W uniformly over an area of 2 m^2. Its intensity is:

  1. 30 W/m^2
  2. 60 W/m^2
  3. 3 W/m^2
  4. 300 W/m^2

Answer: A) 30 W/m^2

Explanation: I = PA = 602 = 30 W/m^2. SLO P-11-D-01 · Application

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5. The Doppler effect is used in 'speed cameras' because:

  1. it measures the vehicle's mass
  2. vehicles emit radar
  3. the frequency shift depends on the speed of the vehicle
  4. the shift depends only on colour

Answer: C) the frequency shift depends on the speed of the vehicle

Explanation: Delta f propto v. SLO P-11-D-04 · Understanding

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6. A foetal heart monitor uses:

  1. X-rays
  2. Doppler ultrasound
  3. gamma rays
  4. infrared

Answer: B) Doppler ultrasound

Explanation: Safe for babies. SLO P-11-D-04 · Understanding

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7. A wave delivers a power of 1000 W uniformly over an area of 4 m^2. Its intensity is:

  1. 2500 W/m^2
  2. 500 W/m^2
  3. 25 W/m^2
  4. 250 W/m^2

Answer: D) 250 W/m^2

Explanation: I = PA = 10004 = 250 W/m^2. SLO P-11-D-01 · Application

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8. Satellite navigation systems can use the Doppler shift of satellite signals to:

  1. increase signal strength
  2. measure gravity
  3. find the position and speed of a receiver
  4. charge the satellite

Answer: C) find the position and speed of a receiver

Explanation: Early satellite tracking used Doppler. SLO P-11-D-04 · Understanding

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9. Police radar speed guns measure a car's speed using:

  1. the car's engine noise
  2. the brightness of the car
  3. the Doppler shift of reflected radio (microwave) waves
  4. the time for sound to travel

Answer: C) the Doppler shift of reflected radio (microwave) waves

Explanation: Shift ∝ speed. SLO P-11-D-04 · Understanding

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10. The Doppler effect happens because the moving source:

  1. changes its frequency
  2. emits successive wavefronts from different positions
  3. creates more waves
  4. changes the speed of sound

Answer: B) emits successive wavefronts from different positions

Explanation: The spacing of wavefronts changes. SLO P-11-D-02 · Understanding

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11. The evidence that the universe is expanding comes from:

  1. blue shift of nearby stars
  2. the brightness of the Sun
  3. the red shift of distant galaxies
  4. the Moon's phases

Answer: C) the red shift of distant galaxies

Explanation: Hubble's observations. SLO P-11-D-04 · Understanding

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12. As an ambulance moves away from a stationary listener, the pitch heard is:

  1. higher than the actual frequency
  2. zero
  3. lower than the actual frequency
  4. unchanged

Answer: C) lower than the actual frequency

Explanation: Wavefronts are stretched out behind the source. SLO P-11-D-02 · Understanding

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13. A planet orbiting a distant star can be detected by:

  1. sound waves from the planet
  2. the periodic Doppler shift ('wobble') of the star's light
  3. measuring the star's mass on a balance
  4. seeing the planet directly

Answer: B) the periodic Doppler shift ('wobble') of the star's light

Explanation: Radial velocity method. SLO P-11-D-04 · Understanding

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14. A point source radiates 40 W of sound equally in all directions. The intensity 2 m away is:

  1. 0.796 W/m^2
  2. 3.18 W/m^2
  3. 1.59 W/m^2
  4. 2.5 W/m^2

Answer: A) 0.796 W/m^2

Explanation: I = P4pi r^2 = 404pi(2)^2 = 0.796 W/m^2. SLO P-11-D-01 · Application

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15. A source of frequency 500 Hz moves at 20 m/s away from a stationary observer. The speed of sound is 340 m/s. The observed frequency is:

  1. 500 Hz
  2. 472 Hz
  3. 531 Hz
  4. 425 Hz

Answer: B) 472 Hz

Explanation: fo = fs vv + vs = 500 340340 + 20 = 472 Hz. SLO P-11-D-03 · Application

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16. A source of frequency 1000 Hz moves at 30 m/s towards a stationary observer. The speed of sound is 330 m/s. The observed frequency is:

  1. 917 Hz
  2. 1000 Hz
  3. 1100 Hz
  4. 1210 Hz

Answer: C) 1100 Hz

Explanation: fo = fs vv - vs = 1000 330330 - 30 = 1100 Hz. SLO P-11-D-03 · Application

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17. If vs is much smaller than v, the fractional change in frequency is approximately:

  1. vs v
  2. vvs
  3. vsv
  4. vs^2v^2

Answer: C) vsv

Explanation: Delta ff approx vsv. SLO P-11-D-03 · Application

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18. A wave delivers a power of 0.5 W uniformly over an area of 1 10^-4 m^2. Its intensity is:

  1. 10,000 W/m^2
  2. 50,000 W/m^2
  3. 5000 W/m^2
  4. 500 W/m^2

Answer: C) 5000 W/m^2

Explanation: I = PA = 0.51 10^-4 = 5000 W/m^2. SLO P-11-D-01 · Application

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19. A progressive wave has intensity 8 W/m^2. If its amplitude is multiplied by 0.25, its new intensity is:

  1. 0.5 W/m^2
  2. 5 W/m^2
  3. 0.25 W/m^2
  4. 1 W/m^2

Answer: A) 0.5 W/m^2

Explanation: I propto A^2, so I = 8 (0.25)^2 = 0.5 W/m^2. SLO P-11-D-01 · Application

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20. The Doppler effect is:

  1. the apparent change in frequency when a source moves relative to an observer
  2. the bending of waves
  3. a change in the speed of sound
  4. the reflection of sound

Answer: A) the apparent change in frequency when a source moves relative to an observer

Explanation: Frequency observed ≠ frequency emitted. SLO P-11-D-02 · Understanding

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21. A source of frequency 600 Hz moves at 34 m/s towards a stationary observer. The speed of sound is 340 m/s. The observed frequency is:

  1. 545 Hz
  2. 733 Hz
  3. 667 Hz
  4. 600 Hz

Answer: C) 667 Hz

Explanation: fo = fs vv - vs = 600 340340 - 34 = 667 Hz. SLO P-11-D-03 · Application

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22. When a source of sound moves, the speed of sound in still air:

  1. increases in front
  2. decreases in front
  3. doubles
  4. does not change

Answer: D) does not change

Explanation: Speed depends on the medium, not the source. SLO P-11-D-02 · Understanding

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23. In a radar speed trap, a car moving towards the radar returns waves of:

  1. the same frequency
  2. no frequency
  3. lower frequency
  4. higher frequency than emitted

Answer: D) higher frequency than emitted

Explanation: Approaching reflector. SLO P-11-D-04 · Understanding

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24. Sonar on ships uses the Doppler effect to detect:

  1. the depth only
  2. salinity
  3. water temperature
  4. the speed of moving submarines or shoals of fish

Answer: D) the speed of moving submarines or shoals of fish

Explanation: Echo frequency is shifted by motion. SLO P-11-D-04 · Understanding

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25. The 'red shift' of distant galaxies shows that they are:

  1. stationary
  2. moving towards us
  3. getting hotter
  4. moving away from us

Answer: D) moving away from us

Explanation: Longer wavelength observed. SLO P-11-D-02 · Understanding

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