Physics questions, chapter 5

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Waves

46 marks. Use Practice to check each answer as you go, or Exam to answer everything against the clock first.

Section A: objective questions

12 marks

Choose the best answer for each question.

Question 1

What does a wave transfer from one place to another?

Matter only
Energy only
Both energy and matter
Neither energy nor matter

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Waves transfer energy without transferring the matter of the medium (a ribbon on a slinky only vibrates on the spot).

For the teacher: Remembering, low difficulty, standard 5.1

Textbook p. 173

Question 2

Which statement describes a stationary wave?

It does not need a medium
Its particles do not vibrate
Its wave profile moves away from the source
Its wave profile does not move with time

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In a stationary wave (e.g. a plucked guitar string) the wave profile does not propagate; particles still vibrate.

For the teacher: Understanding, low difficulty, standard 5.1

Textbook p. 175

Question 3

Sound is a longitudinal wave. How do the air particles vibrate?

Parallel to the direction of propagation of the wave
Perpendicular to the direction of propagation of the wave
In circles around the source
They travel with the wave from the source to the listener

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In a longitudinal wave the particles vibrate parallel to the direction in which the wave moves, forming compressions and rarefactions.

For the teacher: Understanding, low difficulty, standard 5.1

Textbook p. 177

Question 4

Diagram 1 shows a graph of displacement against time for a wave. What is the frequency of the wave?

Displacement-time graph, amplitude 4 cm, one complete cycle in 0.50 s — Diagram 1

0.5 Hz
1.0 Hz
2.0 Hz
4.0 Hz

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One cycle takes T = 0.50 s, so f = 1/T = 1 ÷ 0.50 = 2.0 Hz.

For the teacher: Applying, low difficulty, standard 5.1

Textbook p. 181

Question 5

Diagram 2 shows two graphs for the same wave. What is the speed of the wave?

Displacement-time graph, period 0.50 s — Graph of displacement against time

Displacement-distance graph, wavelength 20 cm — Graph of displacement against time

10 cm s⁻¹
40 cm s⁻¹
80 cm s⁻¹
160 cm s⁻¹

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λ = 20 cm and T = 0.50 s, so v = λ/T = 20 ÷ 0.50 = 40 cm s⁻¹ (or f = 2 Hz, v = 2 × 20).

For the teacher: Analysing, medium difficulty, standard 5.1

Textbook p. 181

Question 6

A driver pendulum of length 40 cm is hung on the same string as four other pendulums: P (20 cm), Q (30 cm), R (50 cm) and S (40 cm). It is set swinging. Which pendulum oscillates with the largest amplitude?

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S has the same length, hence the same natural frequency, as the driver. It is in resonance and oscillates with maximum amplitude.

For the teacher: Applying, low difficulty, standard 5.2

Textbook p. 187

Question 7

Plane water waves of wavelength 2 cm hit a plane reflector. The incident wavefronts make an angle of 30° with the reflector. What are the angle of reflection and the wavelength of the reflected waves?

30° and 2 cm
60° and 2 cm
30° and 4 cm
60° and 1 cm

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The angle between the wavefront and the reflector equals the angle of incidence, 30°. So r = i = 30°. Reflection does not change the wavelength (2 cm).

For the teacher: Applying, low difficulty, standard 5.3

Textbook p. 190

Question 8

Water waves of wavelength 3.0 cm travel at 24 cm s⁻¹ in a deep region. They enter a shallow region where the wavelength is 2.0 cm. What is the speed in the shallow region?

8 cm s⁻¹
12 cm s⁻¹
16 cm s⁻¹
36 cm s⁻¹

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Frequency is constant, so v₁/λ₁ = v₂/λ₂: 24/3.0 = v₂/2.0, hence v₂ = 16 cm s⁻¹.

For the teacher: Applying, medium difficulty, standard 5.4

Textbook p. 200

Question 9

Plane water waves move from a deep region into a shallow region. Which is correct?

The frequency decreases
The wavelength decreases
The waves bend away from the normal
The speed does not change

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In shallow water the speed falls, so the wavelength decreases. Frequency is unchanged and the waves bend towards the normal.

For the teacher: Understanding, low difficulty, standard 5.4

Textbook p. 196

Question 10

In which situation is the diffraction of water waves most obvious?

Wide gap and short wavelength
Wide gap and long wavelength
Narrow gap and short wavelength
Narrow gap and long wavelength

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Diffraction is more obvious when the gap is narrower and when the wavelength is longer.

For the teacher: Understanding, low difficulty, standard 5.5

Textbook p. 208

Question 11

In a Young's double-slit experiment the slit separation is 0.50 mm, the screen is 1.2 m from the slits and adjacent bright fringes are 1.5 mm apart. What is the wavelength of the light?

6.25 × 10⁻¹⁰ m
6.25 × 10⁻⁴ m
6.25 × 10⁻⁷ m
6.25 × 10⁻¹ m

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λ = ax/D = (0.50 × 10⁻³)(1.5 × 10⁻³) ÷ 1.2 = 6.25 × 10⁻⁷ m. Convert mm to m first.

For the teacher: Applying, high difficulty, standard 5.6

Textbook p. 217

Question 12

Which electromagnetic wave carries the most energy?

Gamma ray
X-ray
Ultraviolet
Microwave

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Energy is proportional to frequency. Gamma rays have the highest frequency in the spectrum.

For the teacher: Remembering, low difficulty, standard 5.7

Textbook p. 221

Section B: structured questions

34 marks

Answer all questions. Write your answers in the spaces provided.

Question 13

Diagram 3 shows a wave moving through a spring. P, Q and R are compressions.

Longitudinal wave in a spring with compressions P, Q and R; X marks a region between Q and R; P to R is 8.0 cm — Diagram 3

[1 mark] (a)What type of wave is shown?

ATransverseBLongitudinal

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Longitudinal

The coils vibrate parallel to the direction of the wave.

[1 mark] (b)Name the region marked X.

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Rarefaction1

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[2 marks] (c)Find the wavelength of the wave.

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P to R is two wavelengths1
λ = 8.0 ÷ 2 = 4.0 cm1

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[2 marks] (d)The frequency of the wave is 50 Hz. Calculate its speed.

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v = fλ = 50 × 4.0 (substitution)1
= 200 cm s⁻¹ (2.0 m s⁻¹)1

Tick each point you got.

For the teacher: Understanding, low difficulty, standard 5.1

Textbook p. 177

Question 14

Diagram 4 shows the graph of displacement against time for a simple pendulum.

Displacement-time graph of an oscillation whose amplitude decreases each cycle — Diagram 4

[1 mark] (a)Name the phenomenon shown by the graph.

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Damping1

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[2 marks] (b)State what happens to (i) the amplitude and (ii) the frequency of the pendulum.

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Amplitude decreases with time1
Frequency remains constant1

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[1 mark] (c)Explain why the amplitude decreases.

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The pendulum loses energy to overcome air resistance (and friction at the support)1

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For the teacher: Understanding, low difficulty, standard 5.2

Textbook p. 185

Question 15

Diagram 5 shows plane water waves of wavelength 2.0 cm and frequency 8.0 Hz striking a plane reflector. The angle of incidence is 40°.

Plane wavefronts striking a plane reflector; the angle between the incident ray and the normal is 40 degrees — Diagram 5

[1 mark] (a)State the angle of reflection.

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40°1

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[2 marks] (b)Calculate the speed of the reflected waves.

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Speed does not change; v = fλ = 8.0 × 2.01
= 16 cm s⁻¹1

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[2 marks] (c)A ship sends an ultrasonic pulse to the seabed and receives the echo 0.10 s later. The speed of the pulse in water is 1 500 m s⁻¹. Calculate the depth of the sea.

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2d = vt = 1 500 × 0.10 = 150 m1
d = 75 m1

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For the teacher: Applying, low difficulty, standard 5.3

Textbook p. 193

Question 16

Diagram 6 shows plane water waves of frequency 10 Hz moving from a deep region P into a shallow region Q. The wavelength in P is 3.0 cm and in Q is 1.8 cm.

Plane wavefronts crossing a boundary from deep region P to shallow region Q; wavefronts in Q are closer together and bend towards the normal — Diagram 6

[1 mark] (a)Calculate the speed of the waves in region P.

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v = fλ = 10 × 3.0 = 30 cm s⁻¹1

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[2 marks] (b)Calculate the speed of the waves in region Q.

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v = fλ = 10 × 1.8 (frequency does not change)1
= 18 cm s⁻¹1

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[2 marks] (d)Explain why the waves bend towards the normal in region Q.

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Speed of the waves decreases in the shallow region1
The change in speed causes the direction of propagation to change (refraction), towards the normal1

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For the teacher: Applying, low difficulty, standard 5.4

Textbook p. 200

Question 17

Diagram 7 shows plane water waves passing through a narrow gap in a barrier.

Plane wavefronts passing through a narrow gap in a barrier and spreading out as circular wavefronts — Diagram 7

[1 mark] (a)Name the phenomenon shown.

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Diffraction1

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[2 marks] (b)Compare the wavelength of the waves before and after the gap. Give a reason.

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Wavelength is the same (no change)1
Speed and frequency of the waves do not change1

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[2 marks] (c)State and explain what happens to the amplitude after the gap.

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Amplitude decreases1
Energy is spread over a wider region1

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For the teacher: Creating, high difficulty, standard 5.5

Textbook p. 208

Question 18

Diagram 8 shows the interference pattern formed by two coherent sources, S₁ and S₂, in a ripple tank. P lies on an antinodal line and R lies on a nodal line.

Circular waves from sources S1 and S2 with antinodal lines and nodal lines; point P on an antinodal line and point R on a nodal line — Diagram 8

[1 mark] (a)What is meant by coherent sources?

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Sources which produce waves of the same frequency and with a constant phase difference1

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[2 marks] (b)State the type of interference at P and explain how it is formed.

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Constructive interference1
A crest from S₁ meets a crest from S₂ (trough meets trough), giving a larger amplitude1

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[2 marks] (c)What is the displacement of the water at R? Explain your answer.

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Zero1
Crest from one source meets trough from the other (destructive interference)1

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For the teacher: Analysing, medium difficulty, standard 5.6

Textbook p. 216

Question 19

Electromagnetic waves are used in many ways in medicine and daily life.

[1 mark] (a)Arrange microwave, ultraviolet, X-ray and infrared in order of increasing frequency.

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Microwave, infrared, ultraviolet, X-ray1

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[2 marks] (b)Green light of wavelength 540 nm in air (speed 3.00 × 10⁸ m s⁻¹) passes into a liquid where its speed is 2.25 × 10⁸ m s⁻¹. Calculate its wavelength in the liquid.

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Frequency is unchanged, so λ₂ = 540 × (2.25 ÷ 3.00)1
= 405 nm (4.05 × 10⁻⁷ m)1

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[1 mark] (c)Hospital staff must leave the room while gamma rays sterilise surgical instruments. Explain why.

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Gamma rays carry a very large amount of energy / have the highest frequency and can damage living cells1

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For the teacher: Evaluating, medium difficulty, standard 5.7

Textbook p. 221

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