OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(g) Doppler effect in ultrasound; speed of blood in the patient; Δf/f =…
Full specification wording
Using ultrasound: (g) Doppler effect in ultrasound; speed of blood in the patient; Δf/f = (2v cos θ)/c for determining the speed v of blood.
The Doppler effect occurs when ultrasound is reflected from moving blood cells.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 8 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The Doppler effect occurs when ultrasound is reflected from moving blood cells. The frequency of the reflected wave is shifted by an amount Δf compared with the transmitted frequency f. The speed v of the blood can be found using Δf/f = (2v cos θ)/c, where c is the speed of ultrasound in blood and θ is the angle between the ultrasound beam and the direction of blood flow. The factor 2 arises because the ultrasound is Doppler shifted twice: once on the way to the moving cells and once on the way back. The cos θ term accounts for the component of blood velocity along the beam.
What earns the marks
- State that the Doppler effect causes a change in frequency when ultrasound is reflected from moving blood cells.
- Use the equation Δf/f = (2v cos θ)/c to determine the speed v of blood.
- Explain that the factor 2 arises because the ultrasound is Doppler shifted twice.
- Explain that θ is the angle between the ultrasound beam and the direction of blood flow.
- Recognise that the measured frequency shift depends on cos θ, so the beam should be as parallel as possible to the flow for maximum sensitivity.
Common mistakes and how to put them right
Forgetting the factor 2 in the equation.
Put it right: The correct form has 2v cos θ in the numerator.
Using the speed of sound in air instead of the speed of ultrasound in blood.
Put it right: c must be the speed in the medium being scanned.
Confusing θ with the angle of reflection.
Put it right: θ is the angle between the beam and the direction of blood flow.
Assuming that a larger angle θ gives a larger frequency shift.
Put it right: In fact, cos θ decreases as θ increases, so the shift decreases.
Exam tips
- Check that the angle θ is measured between the beam and the direction of flow, not between the beam and the vessel wall.
- If the beam is perpendicular to the flow, cos 90° = 0, so no Doppler shift is detected.
- Use consistent units: Δf and f in Hz, v and c in m s⁻¹.
- In multiple-choice questions, eliminate options that omit the factor 2 or use sin θ instead of cos θ.
Try the lesson
The start of the Using ultrasound: (g) Doppler effect in ultrasound; speed of blood in the patient; Δf/f = (2v cos θ)/c for determining the speed v of blood. lesson, made from checked teaching notes for this specification point.
Listen
The Doppler effect occurs when ultrasound is reflected from moving blood cells. The frequency of the reflected wave is shifted by an amount Δf compared with the transmitted frequency f. The speed v of the blood can be found using Δf/f = (2v cos θ)/c, where c is the speed of ultrasound in blood and θ is the angle between the ultrasound beam and the direction of blood flow. The factor 2 arises because the ultrasound is Doppler shifted twice: once on the way to the moving cells and once on the way back. The cos θ term accounts for the component of blood velocity along the beam. A strong answer earns marks for these points. State that the Doppler effect causes a change in frequency when ultrasound is reflected from moving blood cells. Use the equation Δf/f = (2v cos θ)/c to determine the speed v of blood. Explain that the factor 2 arises because the ultrasound is Doppler shifted twice. Explain that θ is the angle between the ultrasound beam and the direction of blood flow. Exam tip: Check that the angle θ is measured between the beam and the direction of flow, not between the beam and the vessel wall.
See it
Key words
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Doppler effect
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Blood flow measurement
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Angle dependence of Doppler shift
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See every definition
- Doppler effect
- Occurs when ultrasound is reflected from moving blood cells, so the frequency of the reflected wave is shifted by an amount Δf compared with the transmitted frequency f.
- Blood flow measurement
- The speed v of the blood can be found using Δf/f = (2v cos θ)/c, where c is the speed of ultrasound in blood.
- Angle dependence of Doppler shift
- The measured frequency shift depends on cos θ, so the beam should be as parallel as possible to the flow for maximum sensitivity.
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