IB Diploma Physics chapter 3: Wave behaviour MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

50 exam-style questions on Wave behaviour, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

50 questions · 5 topics · 10 per test

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What chapter 3 covers

Ten Wave behaviour questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. Glass has a refractive index of 1.5. What is the speed of light in the glass? (c = 3.0 × 108 m s−1)

  1. A
    3.0 × 108 m s−1
  2. B
    4.5 × 108 m s−1
  3. C
    2.0 × 108 m s−1
  4. D
    1.5 × 108 m s−1
Show answer

Answer: C. n = c ÷ v, so v = 3.0 × 108 ÷ 1.5 = 2.0 × 108 m s−1. Light always travels slower in a medium than in a vacuum, so 4.5 × 108 is impossible.

2. You can hear someone talking in the next room through an open doorway but cannot see them. What is the main reason?

  1. A
    light cannot reflect off the walls of a room
  2. B
    sound travels faster than light through a doorway
  3. C
    sound wavelengths are similar to the doorway width, so sound diffracts a lot; light wavelengths are far too small
  4. D
    sound is a transverse wave and light is longitudinal
Show answer

Answer: C. Sound has wavelengths of centimetres to metres, comparable to a doorway, so it spreads round the edges. Light's wavelength (about 500 nm) is far too small for noticeable diffraction there.

3. What is the distance between two adjacent nodes on a standing wave of wavelength λ?

  1. A
    λ
  2. B
    2λ
  3. C
    λ/4
  4. D
    λ/2
Show answer

Answer: D. Nodes are half a wavelength apart, as are antinodes. The distance from a node to the nearest antinode is λ/4.

4. On a displacement–time graph for one point on a travelling wave, what is the time between two adjacent peaks?

  1. A
    the wavelength
  2. B
    the frequency
  3. C
    the period
  4. D
    the amplitude
Show answer

Answer: C. A displacement–time graph repeats every period T. On a displacement–distance graph the peak-to-peak spacing is the wavelength instead.

5. A string 0.80 m long is fixed at both ends. Waves travel on it at 240 m s−1. What is the frequency of the first harmonic?

  1. A
    75 Hz
  2. B
    150 Hz
  3. C
    190 Hz
  4. D
    300 Hz
Show answer

Answer: B. For the first harmonic the string is half a wavelength long: λ = 2L = 1.6 m. f = v ÷ λ = 240 ÷ 1.6 = 150 Hz. 300 Hz takes λ = L.

6. Spectral lines from a distant galaxy are redshifted. What does this show?

  1. A
    the galaxy is moving away, so the received wavelengths are shorter
  2. B
    the galaxy is moving away, so the received wavelengths are longer
  3. C
    the galaxy is hotter than our Sun
  4. D
    the galaxy is moving towards us, so the received wavelengths are longer
Show answer

Answer: B. Redshift means an increase in wavelength (towards the red), caused by a source receding from the observer. An approaching source gives blueshift.

7. An ambulance with its siren on drives past a stationary observer. How does the observed frequency compare with the emitted frequency?

  1. A
    lower while it approaches, higher while it moves away
  2. B
    the same, but the sound is louder while it approaches
  3. C
    higher both while it approaches and while it moves away
  4. D
    higher while it approaches, lower while it moves away
Show answer

Answer: D. An approaching source squeezes the wavefronts together (shorter wavelength, higher frequency); a receding source spreads them out (lower frequency).

8. An observer moves towards a stationary source of sound. Why is the frequency heard higher than the emitted frequency?

  1. A
    the wavelength of the sound in the air is shorter
  2. B
    the speed of sound in the air increases
  3. C
    the source emits a higher frequency
  4. D
    the observer meets wavefronts more often each second
Show answer

Answer: D. For a moving observer the wavelength in the air is unchanged, and so is the speed of sound relative to the air. The observer simply crosses more wavefronts per second.

9. A simple pendulum of length 1.0 m has period T. To what length must it be changed to give a period of 2T?

  1. A
    4.0 m
  2. B
    1.4 m
  3. C
    2.0 m
  4. D
    0.25 m
Show answer

Answer: A. T = 2π√(l/g), so T ∝ √l. Doubling T needs the length multiplied by 2² = 4, giving 4.0 m. 2.0 m assumes T ∝ l.

10. A pipe closed at one end is 0.34 m long. The speed of sound is 340 m s−1. Which of these frequencies can the pipe resonate at?

  1. A
    125 Hz
  2. B
    1000 Hz
  3. C
    750 Hz
  4. D
    500 Hz
Show answer

Answer: C. Closed at one end, the first harmonic has λ = 4L: f = 340 ÷ 1.36 = 250 Hz. Only odd multiples are possible (250, 750, 1250 Hz), so 500 and 1000 Hz cannot occur.

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Ways to practise this chapter

Wave behaviour: questions students ask

How many Wave behaviour MCQs are there?

50, each with the answer and a short explanation, across 5 topics: C.1 Simple harmonic motion, C.2 Wave model, C.3 Wave phenomena, C.4 Standing waves and resonance, C.5 Doppler effect. Every chapter test draws ten of them, so you can take it many times.

Is the IB Diploma Physics chapter 3 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

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