Interference
8.3What the syllabus asks of you
- understand the terms interference and coherence
- show an understanding of experiments that demonstrate two-source interference using water waves in a ripple tank, sound, light and microwaves
- understand the conditions required if two-source interference fringes are to be observed
- recall and use λ = ax / D for double-slit interference using light
Read the notes first
Chapter 8: Interference
Drill this topic
Every question on Interference, untimed, marked as you go.
Make a worksheet
Choose the years and how many questions, then print it with the answers on separate pages.
80 questions
1May / June 2026 · Paper 11 · Q29Show mark scheme →
AThe waves must be electromagnetic waves.
BThe waves must be emitted with a constant phase difference.
CThe waves must be emitted with the same amplitude.
DThe waves must be emitted with the same intensity.
Mark scheme
2May / June 2026 · Paper 11 · Q30Show mark scheme →
Aslit separation doubled, distance between slits and screen unchanged
Bslit separation unchanged, distance between slits and screen doubled
Cslit separation halved, distance between slits and screen doubled
Dslit separation doubled, distance between slits and screen halved
Mark scheme
3May / June 2026 · Paper 12 · Q29Show mark scheme →
A0 m
B0.20 m
C0.40 m
D0.80 m
Mark scheme
4May / June 2026 · Paper 13 · Q27Show mark scheme →
AA dark fringe is formed where none of the light reaches the screen because it was blocked at the double slit.
BA dark fringe is formed where the waves of light from the two slits do not overlap and so cannot superpose.
CA dark fringe is formed where a trough from one wave of light meets a trough from the other wave of light.
DA dark fringe is formed where the two waves of light meet and have a phase difference of 180°.
Mark scheme
5May / June 2026 · Paper 14 · Q29Show mark scheme →
A0.080 mm
B0.35 mm
C0.72 mm
D0.80 mm
Mark scheme
6Oct / Nov 2025 · Paper 11 · Q28Show mark scheme →
A8.5 × 10⁻³ m
B2.4 × 10⁻⁴ m
C1.9 × 10⁻⁴ m
D4.8 × 10⁻⁵ m
Mark scheme
7Oct / Nov 2025 · Paper 12 · Q31Show mark scheme →
Adiffraction of the sound waves
BDoppler shift of the sound waves
Cinterference of the sound waves
Dreflection of the sound waves
Mark scheme
8Oct / Nov 2025 · Paper 13 · Q28Show mark scheme →
A8.5 × 10⁻³ m
B2.4 × 10⁻⁴ m
C1.9 × 10⁻⁴ m
D4.8 × 10⁻⁵ m
Mark scheme
9Oct / Nov 2025 · Paper 14 · Q31Show mark scheme →
AThe two waves are emitted with the same frequency.
BThe two waves are emitted with constant phase difference.
CThe two waves are emitted with the same intensity.
DThe two waves are emitted with zero phase difference.
Mark scheme
10May / June 2025 · Paper 12 · Q31Show mark scheme →
A1.2 mm
B2.7 mm
C4.5 mm
D5.4 mm
Mark scheme
11May / June 2025 · Paper 13 · Q28Show mark scheme →
A10 cm
B16 cm
C20 cm
D40 cm
Mark scheme
12May / June 2025 · Paper 13 · Q31Show mark scheme →
Aincreasing the distance between the sources and the screen
Bincreasing the distance between the two sources
Cincreasing the frequency of the light
Dincreasing the intensity of the light
Mark scheme
13May / June 2025 · Paper 14 · Q27Show mark scheme →
ADecrease the distance from the double slit to the screen.
BDecrease the separation of the slits.
CReplace the red light with blue light.
DReplace the red light with green light.
Mark scheme
14Feb / March 2025 · Paper 12 · Q27Show mark scheme →
AThe waves must be polarised.
BThe waves must not be polarised.
CThe waves must be coherent.
DThe waves must have equal amplitudes.
Mark scheme
15Oct / Nov 2024 · Paper 11 · Q27Show mark scheme →
A1, 2 and 3
B1 and 2 only
C1 and 3 only
D2 and 3 only
Mark scheme
16Oct / Nov 2024 · Paper 11 · Q28Show mark scheme →
A0.44I
B1.5I
C2.3I
D3.0I
Mark scheme
17Oct / Nov 2024 · Paper 13 · Q29Show mark scheme →
A4.0 mm
B10 mm
C25 mm
D50 mm
Mark scheme
18May / June 2024 · Paper 11 · Q30Show mark scheme →
AThere is a constant difference in the path lengths YZ and XZ.
BThere is a constant difference in phase between the two waves at Z.
CThere is a constant non-zero difference in frequency of the two waves at Z.
DThere is a constant non-zero difference in amplitude of the two waves at Z.
Mark scheme
19May / June 2024 · Paper 12 · Q28Show mark scheme →
A0.33 m
B0.67 m
C0.75 m
D1.50 m
Mark scheme
20Feb / March 2024 · Paper 12 · Q30Show mark scheme →
AThe waves are emitted in phase.
BThe waves are emitted and move in opposite directions.
CThe waves are emitted with a constant phase difference.
DThe waves are emitted with the same amplitude.
Mark scheme
21Feb / March 2024 · Paper 12 · Q31Show mark scheme →
Athe distance p from the laser to the double slit
Bthe distance q from the double slit to the screen
Cthe separation r of the slits
Dthe wavelength of the light from the laser
Mark scheme
22Oct / Nov 2023 · Paper 11 · Q28Show mark scheme →
ALight from the two sources is emitted 180° out of phase and the path difference to P is ½λ.
BLight from the two sources is emitted in phase and the path difference to P is λ.
CLight from the two sources is emitted 90° out of phase and the path difference to P is λ.
DLight from the two sources is emitted in phase and the path difference to P is ½λ.
Mark scheme
23Oct / Nov 2023 · Paper 12 · Q28Show mark scheme →
AGraph A: x against a, decreasing curve from high x at small a, asymptotic to a-axis
BGraph B: x against a, straight line decreasing from positive x-intercept to zero at positive a
CGraph C: x against a, straight line through origin with positive gradient
DGraph D: x against a, increasing curve through origin, concave up
Mark scheme
24Oct / Nov 2023 · Paper 13 · Q28Show mark scheme →
AGraph A
BGraph B
CGraph C
DGraph D
Mark scheme
25May / June 2023 · Paper 11 · Q28Show mark scheme →
A1.4 m
B2.8 m
C7.0 m
D14 m
Mark scheme
26May / June 2023 · Paper 12 · Q28Show mark scheme →
A\frac{I}{\sqrt{2}}
B\frac{I}{2}
C\frac{I}{2\sqrt{2}}
D\frac{I}{4}
Mark scheme
27May / June 2023 · Paper 13 · Q28Show mark scheme →
AThey have a constant phase difference at the point.
BThey have the same amplitude at the point.
CThey have the same frequency.
DThey travel at the same speed.
Mark scheme
28Feb / March 2023 · Paper 12 · Q28Show mark scheme →
AThey are in phase.
BThey are transverse waves.
CThey have a constant phase difference.
DThey have the same amplitude.
Mark scheme
29Oct / Nov 2022 · Paper 11 · Q29Show mark scheme →
AThe bright fringes and the dark fringes all become brighter.
BThe bright fringes and the dark fringes all become darker.
CThe bright fringes become brighter and the dark fringes become darker.
DThe bright fringes become darker and the dark fringes become brighter.
Mark scheme
30Oct / Nov 2022 · Paper 12 · Q29Show mark scheme →
A2.0 × 10⁻⁷ m
B4.0 × 10⁻⁷ m
C6.0 × 10⁻⁷ m
D8.0 × 10⁻⁷ m
Mark scheme
31Oct / Nov 2022 · Paper 13 · Q28Show mark scheme →
AGraph A
BGraph B
CGraph C
DGraph D
Mark scheme
32May / June 2022 · Paper 11 · Q29Show mark scheme →
AIncrease the brightness of the light source.
BIncrease the distance between the slits and the screen.
CIncrease the distance between the two slits.
DIncrease the frequency of the light used.
Mark scheme
33May / June 2022 · Paper 12 · Q29Show mark scheme →
AA
BB
CC
DD
Mark scheme
34May / June 2022 · Paper 13 · Q30Show mark scheme →
Ax/2
Bx
C2x
D4x
Mark scheme
35Feb / March 2022 · Paper 12 · Q24Show mark scheme →
AThe two waves must be emitted from their sources with the same intensity.
BThe two waves must be in phase with each other at the point.
CThe two waves must be travelling in opposite directions.
DThe two waves must have the same amplitude at the point.
Mark scheme
36Feb / March 2022 · Paper 12 · Q27Show mark scheme →
Adistance OY / m 1.25, distance XY / m 3.50
Bdistance OY / m 2.00, distance XY / m 2.75
Cdistance OY / m 2.75, distance XY / m 2.00
Ddistance OY / m 3.25, distance XY / m 1.50
Mark scheme
37Oct / Nov 2021 · Paper 11 · Q28Show mark scheme →
AS₂P – S₁P = nλ/2
BS₂P – S₁P = nλ
CS₂P – S₁P = (n – ½)λ
DS₂P – S₁P = (n + ½)λ
Mark scheme
38Oct / Nov 2021 · Paper 12 · Q29Show mark scheme →
AThey must be coherent and of the same amplitude.
BThey must be coherent but not necessarily of the same amplitude.
CThey must be of the same amplitude but not necessarily coherent.
DThey must not be coherent or of the same amplitude.
Mark scheme
39Oct / Nov 2021 · Paper 13 · Q29Show mark scheme →
A0.25 m
B0.40 m
C2.5 m
D4.0 m
Mark scheme
40May / June 2021 · Paper 11 · Q28Show mark scheme →
Adecrease the distance p
Bdecrease the distance q
Cdecrease the distance r
Ddecrease the wavelength of the light
Mark scheme
41May / June 2021 · Paper 12 · Q28Show mark scheme →
AThe waves are in phase.
BThe waves have a constant phase difference.
CThe waves have the same amplitude.
DThe waves interfere constructively wherever they overlap.
Mark scheme
42May / June 2021 · Paper 13 · Q29Show mark scheme →
A2.3 mm
B3.5 mm
C5.4 mm
D9.0 mm
Mark scheme
43Feb / March 2021 · Paper 12 · Q28Show mark scheme →
Aλ/4
Bλ/2
Cλ
D3λ/2
Mark scheme
44Oct / Nov 2020 · Paper 11 · Q28Show mark scheme →
AThe waves must be emitted in phase.
BThe waves must be emitted with a similar amplitude.
CThe waves must have the same frequency.
DThe waves must have the same wavelength.
Mark scheme
45Oct / Nov 2020 · Paper 12 · Q29Show mark scheme →
A50 cm, 0.5 mm
B50 cm, 5 mm
C5 m, 0.5 mm
D5 m, 5 mm
Mark scheme
46Oct / Nov 2020 · Paper 13 · Q29Show mark scheme →
AChange the source for one emitting blue light.
BReduce the distance between the light source and the slits.
CReduce the distance between the slits to 0.55 mm.
DReduce the intensity of the light source.
Mark scheme
47May / June 2020 · Paper 11 · Q29Show mark scheme →
A440 nm
B540 nm
C650 nm
D900 nm
Mark scheme
48May / June 2020 · Paper 12 · Q29Show mark scheme →
A6.0 × 10³ λ
B3.0 × 10³ λ
C6.7 × 10⁻⁴ λ
D3.3 × 10⁻⁴ λ
Mark scheme
49May / June 2020 · Paper 13 · Q29Show mark scheme →
Aincrease the width of each slit
Bmove the screen nearer to the double slit
Cuse slits that are further apart
Duse red light instead of green light
Mark scheme
50Feb / March 2020 · Paper 12 · Q28Show mark scheme →
Alaser wavelength 200 nm, slit separation 0.10 mm, slit-screen distance 5.0 m
Blaser wavelength 200 nm, slit separation 100 mm, slit-screen distance 1.0 m
Claser wavelength 600 nm, slit separation 0.10 mm, slit-screen distance 5.0 m
Dlaser wavelength 600 nm, slit separation 100 mm, slit-screen distance 1.0 m
Mark scheme
51Oct / Nov 2019 · Paper 12 · Q27Show mark scheme →
AA
BB
CC
DD
Mark scheme
52Oct / Nov 2019 · Paper 13 · Q29Show mark scheme →
A
B
C
D
Mark scheme
53May / June 2019 · Paper 11 · Q29Show mark scheme →
A1 mm
B4 mm
C8 mm
D16 mm
Mark scheme
54May / June 2019 · Paper 12 · Q30Show mark scheme →
AThe frequency of the light from the two lamps can never be the same.
BThe light from the two lamps can never be coherent.
CThe temperature of the filaments of the two lamps used can never be the same.
DThe wavelength of the light from the two lamps must always be different.
Mark scheme
55May / June 2019 · Paper 12 · Q31Show mark scheme →
ADecrease the distance between the two slits.
BDecrease the width of each slit.
CMove the screen closer to the double slit.
DUse light of a higher frequency.
Mark scheme
56May / June 2019 · Paper 13 · Q28Show mark scheme →
ALight from the two sources is emitted 180° out of phase and the path difference to P is ½λ.
BLight from the two sources is emitted in phase and the path difference to P is λ.
CLight from the two sources is emitted 90° out of phase and the path difference to P is λ.
DLight from the two sources is emitted in phase and the path difference to P is ½λ.
Mark scheme
57May / June 2019 · Paper 13 · Q29Show mark scheme →
A1.5 mm
B4.5 mm
C6.0 mm
D9.0 mm
Mark scheme
58Feb / March 2019 · Paper 12 · Q28Show mark scheme →
AGraph A: intensity against distance from P to Q, showing a varying amplitude with a minimum (zero) at P and increasing amplitude towards Q
BGraph B: intensity against distance from P to Q, showing a minimum (zero) at P and roughly constant amplitude oscillations
CGraph C: intensity against distance from P to Q, showing a maximum at P and roughly constant amplitude oscillations
DGraph D: intensity against distance from P to Q, showing a maximum at P and roughly constant amplitude oscillations
Mark scheme
59Oct / Nov 2018 · Paper 11 · Q28Show mark scheme →
AThe lamps are not point sources.
BThe lamps emit light of different amplitudes.
CThe light from the lamps is not coherent.
DThe light from the lamps is white.
Mark scheme
60Oct / Nov 2018 · Paper 12 · Q29Show mark scheme →
Anλ
B½ nλ
C(n + ½)λ
D(2n + ½)λ
Mark scheme
61Oct / Nov 2018 · Paper 13 · Q28Show mark scheme →
Adiffraction of the sound waves
BDoppler shift of the sound waves
Cinterference of the sound waves
Dreflection of the sound waves
Mark scheme
62May / June 2018 · Paper 11 · Q25Show mark scheme →
Adecreasing the distance p
Bdecreasing the distance q
Cdecreasing the distance r
Ddecreasing the wavelength of the light
Mark scheme
63May / June 2018 · Paper 11 · Q26Show mark scheme →
A0
B\frac{x_0}{2}
Cx_0
D2x_0
Mark scheme
64May / June 2018 · Paper 12 · Q29Show mark scheme →
Ato make a narrow beam of light
Bto make the same amplitude of light incident on each slit
Cto provide coherent light
Dto provide monochromatic light
Mark scheme
65May / June 2018 · Paper 13 · Q26Show mark scheme →
A2.3 mm
B3.5 mm
C5.4 mm
D9.0 mm
Mark scheme
66Feb / March 2018 · Paper 12 · Q26Show mark scheme →
A1.8 mm
B2.5 mm
C3.0 mm
D3.6 mm
Mark scheme
67Feb / March 2018 · Paper 12 · Q27Show mark scheme →
A2.0 × 10⁻⁷ m
B4.0 × 10⁻⁷ m
C6.0 × 10⁻⁷ m
D8.0 × 10⁻⁷ m
Mark scheme
68Oct / Nov 2017 · Paper 11 · Q28Show mark scheme →
Aboth RS and XY
BRS only
CXY only
Dneither RS nor XY
Mark scheme
69Oct / Nov 2017 · Paper 12 · Q30Show mark scheme →
A1/4
B1/2
C2
D4
Mark scheme
70Oct / Nov 2017 · Paper 13 · Q30Show mark scheme →
A
B
C
D
Mark scheme
71May / June 2017 · Paper 11 · Q29Show mark scheme →
A1.2 \times 10^{-4}\ \text{m}
B1.6 \times 10^{-4}\ \text{m}
C3.1 \times 10^{-5}\ \text{m}
D3.3 \times 10^{-9}\ \text{m}
Mark scheme
72May / June 2017 · Paper 12 · Q29Show mark scheme →
A\frac{\lambda}{8}
B\frac{\lambda}{4}
C\frac{\lambda}{2}
D\lambda
Mark scheme
73May / June 2017 · Paper 13 · Q28Show mark scheme →
AS₂P – S₁P = nλ/2
BS₂P – S₁P = nλ
CS₂P – S₁P = (n – ½)λ
DS₂P – S₁P = (n + ½)λ
Mark scheme
74Feb / March 2017 · Paper 12 · Q29Show mark scheme →
A0.22 mm
B0.44 mm
C2.2 mm
D4.4 mm
Mark scheme
75Oct / Nov 2016 · Paper 12 · Q27Show mark scheme →
AThe bright fringes and the dark fringes all become brighter.
BThe bright fringes and the dark fringes all become darker.
CThe bright fringes become brighter and the dark fringes become darker.
DThe bright fringes become darker and the dark fringes become brighter.
Mark scheme
76Oct / Nov 2016 · Paper 12 · Q28Show mark scheme →
A1.3 mm
B1.5 mm
C3.0 mm
D6.0 mm
Mark scheme
77May / June 2016 · Paper 11 · Q27Show mark scheme →
A0.33 m
B0.67 m
C0.75 m
D1.50 m
Mark scheme
78May / June 2016 · Paper 12 · Q27Show mark scheme →
AThe amplitudes of the two waves at R must be the same.
BThe distance PQ must be smaller than the wavelength λ.
CThe two waves from the slits must have travelled the same distance to R.
DThe two waves must be in phase at R.
Mark scheme
79May / June 2016 · Paper 12 · Q28Show mark scheme →
Aseparation of fringes: decreases; maximum intensity of dark fringes: no change
Bseparation of fringes: increases; maximum intensity of dark fringes: greater
Cseparation of fringes: no change; maximum intensity of dark fringes: greater
Dseparation of fringes: no change; maximum intensity of dark fringes: no change
Mark scheme
80May / June 2016 · Paper 13 · Q28Show mark scheme →
Acoherence
Bdiffraction
Cinterference
Dreflection
Mark scheme