A-Level · Physics · AQA · Mark scheme decoded

AQA A-Level Physics: Ultrasound in Medical Physics — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Physics specificationlast verified 3 July 2026

The short answer

In medical physics, ultrasound imaging is a crucial technique used for visualizing internal structures of the body. This section covers the reflection and transmission characteristics of sound waves at tissue boundaries, acoustic impedance, attenuation, and the principles of generation and detection of ultrasound pulses using piezoelectric devices.

The question

A sound wave travels from a material with acoustic impedance Z 1 = 1.5 × 10 6 rayls to another material with acoustic impedance Z 2 = 3.0 × 10 6 rayls. Calculate the intensity reflection coefficient (I r ).

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For calculation questions, marks are typically awarded for correct use of formulas, substitution of values, and final answers. For conceptual questions, marks are given for clear explanations, relevant examples, and accurate descriptions.

What the command words demand

Calculate: Perform a numerical calculation using given data and appropriate formulas.
Explain: Provide a detailed account of how or why something happens, including relevant concepts and principles.
Compare: Identify similarities and differences between two or more items or concepts.
Describe: Give a detailed account of the characteristics or features of something.
Discuss: Consider multiple aspects of a topic, providing arguments for and against different viewpoints.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to read the question carefully, perform necessary calculations, and provide detailed explanations.

Identify the given values.0 marks
Z 1 = 1.5 × 10 6 raylsZ 2 = 3.0 × 10 6 rayls
Use the formula for the intensity reflection coefficient.1 mark
I r = (Z 2 - Z 1 ) 2 / (Z 2 + Z 1 ) 2
Substitute the values into the formula.1 mark
I r = (3.0 × 10 6 - 1.5 × 10 6 ) 2 / (3.0 × 10 6 + 1.5 × 10 6 ) 2
Simplify the expression.1 mark
I r = (1.5 × 10 6 ) 2 / (4.5 × 10 6 ) 2
Calculate the final value.1 mark
I r = (2.25 × 10 12 ) / (20.25 × 10 12 )I r = 0.111

Final answer: 0.111

Work through every step correctly and you earn all 4 marks.

Another worked example

A piezoelectric transducer is used to generate an ultrasound wave with a frequency of 5 MHz in soft tissue, where the speed of sound is 1540 m/s and the density is 1000 kg/m 3 . Calculate the acoustic impedance (Z) of the soft tissue.

3 marks

Identify the given values.0 marks
c = 1540 m/sρ = 1000 kg/m 3
Use the formula for acoustic impedance.1 mark
Z = ρc
Substitute the values into the formula.1 mark
Z = 1000 kg/m 3 × 1540 m/s
Calculate the final value.1 mark
Z = 1.54 × 10 6 rayls

Final answer: 1.54 × 10 6 rayls

Work through every step correctly and you earn all 3 marks.

Common mistakes

Confusing acoustic impedance (Z) with the speed of sound (c).
Why it happens: Students may mix up the formulas for acoustic impedance and the speed of sound, leading to incorrect calculations.
Fix: Remember that acoustic impedance (Z) is given by Z = ρc, where ρ is the density and c is the speed of sound. The speed of sound alone does not account for the material's resistance to sound propagation.
Forgetting to square the impedance difference in the intensity reflection coefficient formula.
Why it happens: Students may overlook the squaring step, leading to incorrect values for I r .
Fix: Always use the correct formula: I r = (Z 2 - Z 1 ) 2 / (Z 2 + Z 1 ) 2 . Ensure you square the difference in acoustic impedances.
Misinterpreting A-scans and B-scans as the same type of image.
Why it happens: Students may not fully understand the differences between A-scans and B-scans, leading to confusion in exam questions.
Fix: Remember that an A-scan is a one-dimensional display showing depth information, while a B-scan is a two-dimensional cross-sectional image created using multiple A-scans. Understand the unique features of each type of scan.
Forgetting to consider both absorption and scattering in attenuation calculations.
Why it happens: Students may focus only on one aspect of attenuation, leading to incomplete answers.
Fix: Attenuation is the loss of energy due to both absorption and scattering. Ensure you account for both factors when discussing or calculating attenuation.
Confusing the mechanical index (MI) with the thermal index (TI).
Why it happens: Students may mix up these safety indices, leading to incorrect answers in exam questions.
Fix: The mechanical index (MI) measures the potential for mechanical effects like cavitation, while the thermal index (TI) measures the potential for tissue heating. Ensure you understand and use the correct index for each context.
Overlooking the operator dependence of ultrasound imaging in discussions of disadvantages.
Why it happens: Students may focus on more obvious disadvantages like resolution, leading to incomplete answers.
Fix: Operator dependence is a significant disadvantage of ultrasound imaging. The quality of the image can vary based on the skill and experience of the operator. Always include this point when discussing the limitations of ultrasound.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Intensity Reflection Coefficient	Calculate the fraction of sound intensity reflected at a boundary between two acoustic impedances.	4
Calculate Acoustic Impedance	Use Z = ρc to find the acoustic impedance of soft tissue.	3
Ultrasound Reflection Intensity	Use the reflection coefficient formula with acoustic impedances to find the reflected wave intensity.	5
Compare Ultrasound Scans	Explain how combining multiple A-scans lets a B-scan build a two-dimensional image.	3
Total across these question types		15

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.