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CAIE A-Level Physics AS 8.3 Interference Questions

Practise analysing coherent-source interference, path and phase differences, experimental conditions and quantitative fringe patterns.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • define interference and coherence and identify constructive or destructive regions from phase or path difference
  • interpret two-source interference experiments across water, sound, light or microwaves and state the required conditions
  • calculate double-slit fringe spacing, wavelength, slit separation or screen distance using λ = ax/D

Covered syllabus points

8.3.1—The terms interference and coherence8.3.2—Experiments that demonstrate two-source interference8.3.3—The conditions required if two-source interference fringes are to be8.3.4—Λ = ax / D for double-slit interference using light

Question 1

[Maximum number: 1]

Microwaves are emitted from two sources at points X and Y. The two waves meet at point Z. The diagram shows the paths of the two waves.

Figure for Question 1 — CAIE A-Level Physics AS

The waves emitted from points X and Y are coherent.
What is a direct consequence of the two waves being coherent?

A

There is a constant difference in the path lengths YZ and XZ .

B

There is a constant difference in phase between the two waves at Z.

C

There is a constant non-zero difference in frequency of the two waves at Z.

D

There is a constant non-zero difference in amplitude of the two waves at Z .

Question 2

[Maximum number: 1]

The diagram shows a view from above of a double-slit interference demonstration.
L is a monochromatic light source with a vertical filament. B is a barrier with two narrow vertical slits and S is a screen upon which interference fringes form.

Figure for Question 2 — CAIE A-Level Physics AS

The intensity is I at the point on the screen where the centre of the fringe pattern forms.
When one of the slits is covered, what is the intensity at the same point on the screen?

A

I2\frac{I}{\sqrt{2}}

B

I2\frac{I}{2}

C

I22\frac{I}{2 \sqrt{2}}

D

I4\frac{I}{4}

Question 3

[Maximum number: 1]

Light of wavelength λ\lambda is emitted from two point sources R and S and falls onto a distant screen.

Figure for Question 3 — CAIE A-Level Physics AS

At point P on the screen, the light intensity is zero.
What could explain the zero intensity at P ?

A

Light from the two sources is emitted 180∘180^{\circ} out of phase and the path difference to P is 12λ\frac{1}{2} \lambda.

B

Light from the two sources is emitted in phase and the path difference to P is λ\lambda.

C

Light from the two sources is emitted 90∘90^{\circ} out of phase and the path difference to P is λ\lambda.

D

Light from the two sources is emitted in phase and the path difference to P is 12λ\frac{1}{2} \lambda.

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