Double Slit Interference Past Paper Questions
1.
The diagram shows two identical loudspeakers, A and B, placed 0.75 m apart. Each loudspeaker emits sound of frequency 2000 Hz. not to scale
A
0.75m
B
E
C
D
5.0m
Point C is on a line midway between the speakers and 5.0 m away from the line joining the speakers. A listener at C hears a maximum intensity of sound. If the listener then moves from C to E or D, the sound intensity heard decreases to a minimum. Further movement in the same direction results in the repeated increase and decrease in the sound intensity. speed of sound in air = 330 m s
–1
(a) Explain why the sound intensity
(i) is a maximum at C,
(b)
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(ii) is a minimum at D or E.
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Calculate
(4)
(i) the wavelength of the sound,
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(ii) the distance CE.
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(4)
(Total 8 marks)
Ulverston, Victoria High School 1

2.
A laser is used with a double slit positioned 5.0 m from a white screen. The separation of the slits is 0.25 mm and the wavelength of the laser light is 630 mm.
The screen is removed and a linear air track positioned so that a glider on the air track moves in the same plane that was occupied by the screen. air track laser glider
LDR
5.0 m
A light dependent resistor, LDR, is attached to a glider and connected by loosely hanging leads to a datalogger. When the glider moves at a constant speed, the datalogger records the output voltage from a circuit containing the LDR. Output from the datalogger, plotted against time, is shown below. output voltage
V
2 leads to data logger
V
1
0.25
0.50
0.75
1.0
time/s
(a) (i) Explain why the LDR output voltage varies with the position of the glider.
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(ii) Calculate the separation between two adjacent positions of the glider when the
LDR is under maximum illumination.
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Ulverston, Victoria High School 2

(iii) Use your answer to (a)(ii) and the graph to calculate a speed for the glider consistent with these results.
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(5)
(Total 5 marks)
3.
(a) A double slit interference experiment is set up in a laboratory using a source of yellow monochromatic light of wavelength 5.86 × 10
–7 m. The separation of the two vertical parallel slits is 0.36
mm and the distance from the slits to the plane where the fringes are observed is 1.80m.
(i) Describe the appearance of the fringes.
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(ii) Calculate the fringe separation, and also the angle between the middle of the central fringe and the middle of the second bright fringe.
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(Total 5 marks)
Ulverston, Victoria High School 3

4.
A vertical screen is placed several metres beyond a vertical double slit arrangement illuminated by a laser. The diagram below shows a full-size tracing of the pattern of spots obtained on this screen. The black patches represent red light whilst the spaces between them are dark.
(a) Using the wave theory, explain how the pattern of bright and dark patches is formed.
You may be awarded marks for the quality of written communication provided in your answer.
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(3)
(b) The slit separation was 0.90 mm and the distance between the slits and the screen was 4.2 m.
(i) Calculate the spacing of the bright fringes by taking measurements on the diagram of the tracing.
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(ii) Hence determine the wavelength of the laser light used.
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(4)
(Total 7 marks)
Ulverston, Victoria High School 4