AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Ultrasound waves are partially reflected when they meet a boundary between…
Full specification wording
Waves for detection and exploration (physics only) (HT only): Ultrasound waves are partially reflected when they meet a boundary between two different media.
The key word is partially.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 3 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The key word is partially. When an ultrasound pulse reaches a boundary between two media, some of it is reflected back towards the probe and the rest carries on into the second medium. That is what makes scanning possible: the first boundary returns an echo but does not stop the pulse, so deeper boundaries return their own echoes a little later and each appears as a separate pulse on the detector trace. The greater the difference between the two media, the larger the fraction reflected, which is why a tissue to air or tissue to bone boundary reflects so strongly. It is also why coupling gel is smeared on the skin: it removes the air gap that would otherwise reflect almost the whole pulse at the surface.
What earns the marks
- Saying that part of the pulse is reflected at the boundary and part is transmitted.
- Saying the reflected fraction depends on how different the two media are.
- Explaining that transmission lets deeper boundaries produce their own echoes.
- Explaining that gel removes the air gap which would otherwise reflect almost all of the pulse.
Common mistakes and how to put them right
Saying all the ultrasound is reflected at the first boundary, which would make deeper imaging impossible.
Forgetting the gel, or saying it is used to make the probe slippery rather than to remove the air gap.
Confusing partial reflection with refraction.
Treating several echoes on a trace as coming from the same boundary.
Exam tips
- Write partially reflected rather than reflected; the word partially is often the marking point.
- On an oscilloscope trace, label the transmitted pulse and each echo before you calculate anything.
- Say what happens to the part that is not reflected; answers that ignore it lose the transmission mark.
Try the lesson
The start of the Waves for detection and exploration (physics only) (HT only): Ultrasound waves are partially reflected when they meet a boundary between two different media. lesson, made from checked teaching notes for this specification point.
Have a go first
An ultrasound pulse travelling through a medium reaches a boundary with a different medium. What happens to the pulse at the boundary?
Listen
The key word is partially. When an ultrasound pulse reaches a boundary between two media, some of it is reflected back towards the probe and the rest carries on into the second medium. That is what makes scanning possible: the first boundary returns an echo but does not stop the pulse, so deeper boundaries return their own echoes a little later and each appears as a separate pulse on the detector trace. The greater the difference between the two media, the larger the fraction reflected, which is why a tissue to air or tissue to bone boundary reflects so strongly. It is also why coupling gel is smeared on the skin: it removes the air gap that would otherwise reflect almost the whole pulse at the surface. A strong answer earns marks for these points. saying that part of the pulse is reflected at the boundary and part is transmitted. saying the reflected fraction depends on how different the two media are. explaining that transmission lets deeper boundaries produce their own echoes. explaining that gel removes the air gap which would otherwise reflect almost all of the pulse. Exam tip: Write partially reflected rather than reflected; the word partially is often the marking point.
Key words
Say what each word means before you turn the card.
partial reflection
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boundary between media
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echo pulses
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coupling gel
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See every definition
- partial reflection
- When an ultrasound pulse meets a boundary, some of it is reflected back towards the probe and the rest carries on into the second medium.
- boundary between media
- The place where two different media meet, at which part of an ultrasound pulse is reflected and part is transmitted.
- echo pulses
- Separate pulses on the detector trace, each returned a little later by its own deeper boundary as the pulse passes through layered tissue.
- coupling gel
- A gel smeared on the skin to remove the air gap that would otherwise reflect almost the whole pulse at the surface.
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