Reflection and the pulse-echo technique Edexcel International A Level Physics revision
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In plain words
Shout at a cliff and count the seconds until the echo: you have measured how far away it is. Bats, ships' sonar and hospital ultrasound scanners all do the same thing, with short bursts of sound too high to hear.
4 things to know
- When a wave meets a boundary between two media, some of it is transmitted and some is reflected.
- Pulse-echo: a short pulse is sent out, and the time t until its echo comes back is measured. The pulse travels there and back, so the distance to the reflector is vt ÷ 2.
- The detail that can be seen is limited by the wavelength: things much smaller than the wavelength can't be picked out. A shorter wavelength (a higher frequency) shows finer detail.
- It is also limited by how long each pulse lasts. Pulses must be short, so that echoes from two surfaces close together don't overlap, and spaced out, so that each echo is back before the next pulse leaves.
Worked example
An ultrasound pulse is sent into the body, and the echo from an organ returns 80 µs later. The speed of sound in tissue is 1500 m/s. Find how deep the organ is.
- Distance travelled = vt = 1500 × 80 × 10⁻⁶ = 0.12 m.
- That is there and back.
- Depth = 0.12 ÷ 2 = 0.060 m (6.0 cm).
Tips and tricks
- Halve the distance: the pulse goes there and back.
- Short wavelength for small detail; short pulses for surfaces that are close together.
It lands in your notebook with its questions as flashcards.
Reflection and the pulse-echo technique: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
An echo returns 0.20 s after a pulse is sent. The speed of sound is 340 m/s. How far away is the reflector?
340 × 0.20 ÷ 2.
Why is high-frequency ultrasound used for detailed scans?
Detail much smaller than the wavelength can't be seen.
What happens when a wave meets a boundary between two media?
The echo is the reflected part.
Why must the pulses be short?
Two overlapping echoes can't be told apart.
A scan uses waves of wavelength 0.5 mm. Roughly what is the smallest detail it can show clearly?
About one wavelength.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Reflection and the pulse-echo technique
Edexcel International A Level Physics WPH · 6 marks · papermunch.org
Name ______________________________ Date ______________
A ship's sonar pulse returns from the sea bed after 0.40 s. The speed of sound in sea water is 1500 m/s. Find the depth of the sea.[2]
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300 m.
Ultrasound of frequency 3.0 MHz travels through tissue at 1500 m/s. Find its wavelength.[2]
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5.0 × 10⁻⁴ m (0.50 mm).
Explain why pulses are used rather than a continuous wave.[2]
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So that the time for each echo to return can be measured, and each echo has come back before the next pulse is sent.
Answers: Reflection and the pulse-echo technique
- 1. 300 m.
- 2. 5.0 × 10⁻⁴ m (0.50 mm).
- 3. So that the time for each echo to return can be measured, and each echo has come back before the next pulse is sent.