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Date May 2019 Marks available 3 Reference code 19M.2.HL.TZ1.3
Level Higher level Paper Paper 2 Time zone 1
Command term Determine Question number 3 Adapted from N/A

Question

A beam of microwaves is incident normally on a pair of identical narrow slits S1 and S2.

When a microwave receiver is initially placed at W which is equidistant from the slits, a maximum in intensity is observed. The receiver is then moved towards Z along a line parallel to the slits. Intensity maxima are observed at X and Y with one minimum between them. W, X and Y are consecutive maxima.

Explain why intensity maxima are observed at X and Y.

[2]
a.

The distance from S1 to Y is 1.243 m and the distance from S2 to Y is 1.181 m.

Determine the frequency of the microwaves.

[3]
b.

Outline one reason why the maxima observed at W, X and Y will have different intensities from each other.

[1]
c.

The microwaves emitted by the transmitter are horizontally polarized. The microwave receiver contains a polarizing filter. When the receiver is at position W it detects a maximum intensity.

The receiver is then rotated through 180° about the horizontal dotted line passing through the microwave transmitter. Sketch a graph on the axes provided to show the variation of received intensity with rotation angle.

[2]
d.

Markscheme

two waves superpose/mention of superposition/mention of «constructive» interference ✔

they arrive in phase/there is a path length difference of an integer number of wavelengths ✔

a.

path difference = 0.062 «m»✔

so wavelength = 0.031 «m»✔

frequency = 9.7 × 109 «Hz»✔

Award [2 max] for 4.8 x 109 Hz.

b.

intensity is modulated by a single slit diffraction envelope OR

intensity varies with distance OR points are different distances from the slits ✔

c.

cos2 variation shown ✔

with zero at 90° (by eye) ✔

Award [1 max] for an inverted curve with maximum at 90°.

d.

Examiners report

Many candidates were able to discuss the interference that is taking place in this question, but few were able to fully describe the path length difference. That said, the quality of responses on this type of question seems to have improved over the last few examination sessions with very few candidates simply discussing the crests and troughs of waves.

a.

Many candidates struggled with this question. Few were able to calculate a proper path length difference, and then use that to calculate the wavelength and frequency. Many candidates went down blind paths of trying various equations from the data booklet, and some seemed to believe that the wavelength is just the reciprocal of the frequency.

b.

This is one of many questions on this paper where candidates wrote vague answers that did not clearly connect to physics concepts or include key information. There were many overly simplistic answers like “they are farther away” without specifying what they are farther away from. Candidates should be reminded that their responses should go beyond the obvious and include some evidence of deeper understanding.

c.

This question was generally well answered, with many candidates at least recognizing that the intensity would decrease to zero at 90 degrees. Many struggled with the exact shape of the graph, though, and some drew a graph that extended below zero showing a lack of understanding of what was being graphed.

d.

Syllabus sections

Core » Topic 4: Waves » 4.4 – Wave behaviour
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Core » Topic 4: Waves
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