The shape of the lesson
By the end of the lesson, learners can
- Send a pulse, read the echo: Move the flaw deeper and watch the echo arrive later on the A-scan trace, with the gate marking the target depth.
- Timing turns into distance: Because the pulse travels to the boundary and back, the recorded time is for twice the depth.
- Definition: ultrasound is sound above 20 kHz, beyond human hearing.
- Equation: total distance = v t, so the distance to the reflecting boundary is d = v t / 2.
- Safety: ultrasound is non-ionising, so it is safe where X-rays would not be.
One picture every idea returns to
Ultrasound is sound with a frequency above 20 000 Hz, too high to hear. By sending a pulse and timing the echo, and using distance = speed multiplied by time, it measures depths and images inside objects. The reflected pulse covers twice the depth, so the time is halved to find the depth.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Sound you cannot hear.. Above the top note any human can hear lies a whole band of sound still obeying the same physics. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Send a pulse, read the echo | Move the flaw deeper and watch the echo arrive later on the A-scan trace, with the gate marking the target depth. Project tab 4 of the lab (Ultrasound) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Timing turns into distance | Because the pulse travels to the boundary and back, the recorded time is for twice the depth. Model the worked example Worked example on the board, one line at a time. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Timing turns into distance, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The echo travels to the boundary and back, so the measured time is for twice the depth. | Halve it, or divide the round trip distance by two. Ultrasound is chosen to scan a foetus because, unlike X-rays, it is not ionising and does not damage living cells. |
Ultrasound: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by ultrasound, and give the approximate frequency above which sound is classed as ultrasound.
State two practical uses of ultrasound.
A ship sends a pulse of ultrasound straight down and receives the echo from the seabed 0.50 s later. The speed of sound in seawater is 1500 m/s. Calculate the depth of the seabed.
During a medical scan, a pulse of ultrasound enters the body and reflects from a boundary between two tissues. The echo is received 0.000040 s (40 microseconds) after the pulse is sent. The speed of sound in the tissue is 1500 m/s. Calculate the depth of the boundary below the skin.
Ultrasound, rather than X-rays, is used to scan an unborn baby. Explain why ultrasound is the safer choice.
Explain how a pulse of ultrasound is used to measure the distance to a boundary inside the body, and explain why the distance is half of v times t.
Answer key and marking notes
- Ultrasound is sound with a frequency too high for humans to hear. ✓
- It has a frequency above about 20 000 Hz (20 kHz). ✓
- Medical scanning, for example imaging an unborn baby. ✓
- Sonar or echo sounding, for example finding the depth of water or locating shoals of fish. ✓
Total path: v t = 1500 x 0.50 = 750 m
d = v t / 2 = 750 / 2d = 375 m
The pulse travels down and back, so the depth is half the total path.
Total path: v t = 1500 x 0.000040 = 0.060 m
d = v t / 2 = 0.060 / 2d = 0.030 m (3.0 cm)
A sensible depth inside the body.
- Ultrasound is a sound wave and is non-ionising, so it does not damage cells. ✓
- X-rays are ionising and could damage or mutate cells, which is a serious risk to a developing baby. ✓
- A pulse is sent in and part of it is reflected at the boundary between two different tissues. ✓
- The time for the echo to return is measured, and the total distance travelled is v t. ✓
- The pulse travels to the boundary and back, so this is twice the depth, giving d = v t / 2. ✓
5 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Ultrasound consists of sound waves with a frequency:
- Aabove 20 000 Hz, beyond the upper limit of human hearing
- Bbelow 20 Hz
- Cbetween 20 Hz and 200 Hz
- Dexactly equal to 1000 Hz
A ship sends an ultrasound pulse straight down to the seabed. The reflected pulse returns 0.40 s later. The speed of ultrasound in water is 1500 m/s. How deep is the water?
- A150 m
- B300 m
- C600 m
- D3750 m
An ultrasound pulse reflects off an object underwater and returns 0.20 s after it was sent. The speed of ultrasound in water is 1500 m/s. How far away is the object?
- A75 m
- B150 m
- C300 m
- D7500 m
Ultrasound is preferred over X-rays for scanning an unborn baby because:
- Ait does not ionise cells, so it is safer for the developing baby
- Bit is much cheaper than any other method
- Cit travels faster than light
- Dit produces colour photographs
Besides medical scanning, ultrasound is commonly used for:
- Ameasuring sea depth (sonar) and detecting cracks inside metals
- Bcooking food quickly
- Csending television signals
- Dgenerating electricity
Quiz key
Ultrasound is sound with a frequency above 20 000 Hz (20 kHz), too high for humans to hear. Frequencies below 20 Hz (B) are infrasound, below the range of hearing.
Total distance = v × t = 1500 × 0.40 = 600 m for the round trip, so the depth is half, 300 m. Option C forgets to halve the round-trip distance.
Distance there and back = 1500 × 0.20 = 300 m, so the object is half of that, 150 m, away. Option C forgets to halve.
Ultrasound is non-ionising, so it does not damage the cells of the developing baby, unlike X-rays which are ionising. Safety, not speed or cost, is the key reason here.
Ultrasound reflects from boundaries, so it is used in sonar to measure sea depth and in industry to detect flaws inside metal castings. Cooking and TV signals use other parts of the spectrum.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.