DP Physics (last assessment 2024)

Question 18M.3.HL.TZ1.d.ii

Date	May 2018	Marks available	[Maximum mark: 4]	Reference code	18M.3.HL.TZ1.d.ii
Level	HL	Paper	3	Time zone	TZ1
Command term	Explain	Question number	d.ii	Adapted from	N/A

d.ii.

[Maximum mark: 4]

18M.3.HL.TZ1.d.ii

The table shows the speed of ultrasound and the acoustic impedance for different media.

M18/4/PHYSI/HP3/ENG/TZ1/14.d

The fraction F of the intensity of an ultrasound wave reflected at the boundary between two media having acoustic impedances Z₁ and Z₂ is given by F = $\frac{{{{({{\text{Z}}_1} - {{\text{Z}}_2})}^2}}}{{{{({{\text{Z}}_1} + {{\text{Z}}_2})}^2}}}$ .

Explain, with appropriate calculations, why a gel is used between the transducer and the skin.

[4]

Markscheme

F = $\frac{{{{(1.99 \times {{10}^6} - 4.3 \times {{10}^2})}^2}}}{{{{(1.99 \times {{10}^6} + 4.3 \times {{10}^2})}^2}}}$ = 1.0

F = $\frac{{{{(1.48 \times {{10}^6} - 1.99 \times {{10}^6})}^2}}}{{{{(1.48 \times {{10}^6} + 1.99 \times {{10}^6})}^2}}}$ = 0.02

almost 100% of the ultrasound will be reflected from the air-skin surface OR almost none is transmitted

whereas only 2% will be reflected from the gel-skin surface and so a much greater proportion is transmitted

Need to explain that more is transmitted through gel-skin surface for MP4.

[4 marks]

Syllabus sections

Option C: Imaging

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

Option C: Imaging » Option C: Imaging (Additional higher level option topics)

Question 18M.3.HL.TZ1.d.ii

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