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Date November 2018 Marks available 2 Reference code 18N.3.HL.TZ0.16
Level Higher level Paper Paper 3 Time zone 0 - no time zone
Command term Determine Question number 16 Adapted from N/A

Question

A beam of ultrasound of intensity I0 enters a layer of muscle of thickness 4.1 cm.

The fraction of the intensity that is reflected at a boundary is

(Z1Z2Z1+Z2)2

where Z1 and Z2 are the acoustic impedances of the two media at the boundary. After travelling a distance x in a medium the intensity of ultrasound is reduced by a factor e–μx where μ is the absorption coefficient.

The following data are available.

Acoustic impedance of muscle     = 1.7 × 106 kg m–2 s–1

Acoustic impedance of bone        = 6.3 × 106 kg m–2 s–1

Absorption coefficient of muscle  = 23 m–1

Determine, in terms of I0, the intensity of ultrasound that is incident on the muscle–bone boundary.

[2]
a.

Determine, in terms of I0, the intensity of ultrasound that is reflected at the muscle–bone boundary.

[2]
b.

Determine, in terms of I0, the intensity of ultrasound that returns to the muscle–gel boundary.

[1]
c.

Markscheme

I0e−23 × 0.041 

= 0.39 I

a.

R = «(6.3×1061.7×1066.3×106+1.7×106)2 =» 0.33 ✔

so reflected intensity is 0.33 × 0.39I0 = 0.13I

b.

0.13I× 0.39 = 0.05I0

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Option C: Imaging » Option C: Imaging (Additional higher level option topics) » C.4 – Medical imaging (HL only)
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Option C: Imaging » Option C: Imaging (Additional higher level option topics)
Option C: Imaging

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