Ultrasound

    OCR
    GCSE

    Ultrasound consists of longitudinal sound waves with frequencies above 20,000 Hz, exceeding the upper limit of human hearing. When these waves encounter a boundary between two different media, they undergo partial reflection, with the remainder being transmitted or absorbed. By measuring the time delay of the reflected echo and applying the speed of sound in the specific medium, the distance to the boundary can be calculated. This principle is fundamental to applications such as prenatal scanning, industrial flaw detection, and sonar depth gauging.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining ultrasound as sound waves with a frequency greater than 20,000 Hz (20 kHz)
    • Credit responses that state partial reflection occurs at the boundary between two different media
    • Award 1 mark for calculating distance using distance = speed × time, explicitly halving the time for echo contexts
    • Candidates must explain that the time delay of the reflected pulse is directly proportional to the depth of the boundary
    • Award 1 mark for identifying ultrasound as non-ionising radiation when comparing with X-rays

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the frequency range, but check your calculation: did the pulse travel one way or there and back?"
    • "Use the term 'partial reflection' rather than just 'reflection' to explain how we can see structures at different depths"
    • "Good comparison of safety features; now add detail on the resolution of images to access higher marks"
    • "Remember that ultrasound is a longitudinal wave—mentioning compressions and rarefactions can strengthen your description of the wave nature"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining ultrasound as sound waves with a frequency greater than 20,000 Hz (20 kHz)
    • Credit responses that state partial reflection occurs at the boundary between two different media
    • Award 1 mark for calculating distance using distance = speed × time, explicitly halving the time for echo contexts
    • Candidates must explain that the time delay of the reflected pulse is directly proportional to the depth of the boundary
    • Award 1 mark for identifying ultrasound as non-ionising radiation when comparing with X-rays

    Examiner Tips

    Expert advice for maximising your marks

    • 💡In calculation questions involving sonar or foetal scanning, immediately annotate the time value to check if it represents the 'there and back' journey
    • 💡When explaining medical imaging, explicitly link the 'boundary between media' to the change in density (e.g., muscle to bone) to secure AO1 marks
    • 💡For 6-mark comparison questions (e.g., Ultrasound vs X-rays), structure your answer into: Safety, Image Quality, and Mechanism of Action

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Failing to divide the total time by 2 (or distance by 2) in echo sounding calculations, resulting in a depth value that is double the correct answer
    • Stating that ultrasound travels at the speed of light (3 × 10^8 m/s) rather than the speed of sound in the specific medium
    • Using vague terminology like 'bouncing off' instead of 'reflecting at the boundary' or failing to mention that only *some* of the wave is reflected (partial reflection)

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Definition of ultrasound (>20 kHz)
    Partial reflection at boundaries between media
    Echo sounding and distance calculation
    Medical and industrial applications (imaging and detection)

    Likely Command Words

    How questions on this topic are typically asked

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    Calculate
    Explain
    Describe
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