C.5
Doppler Effect
The Doppler effect for sound and electromagnetic waves, red shift, applications in astronomy and medicine. HL adds solving problems for source/observer velocity.
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C.5 Doppler Effect
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Key Concepts, Doppler Effect
What the Doppler Effect Is
The Doppler effect is the change in observed frequency (or wavelength) of a wave when there is relative motion between the source and the observer. When a source moves towards an observer, successive wavefronts are compressed, so the observer detects a higher frequency (shorter wavelength). When the source moves away, wavefronts are stretched and the observer detects a lower frequency (longer wavelength). The effect applies to all types of wave: sound, light and other electromagnetic radiation. The classic example is an ambulance siren: high pitch as it approaches, lower pitch as it passes and recedes.
Doppler Effect for Electromagnetic Waves and Red Shift
For electromagnetic waves, when the source moves away from the observer (or the observer moves away from the source), the observed wavelength is longer than emitted (shifted towards the red end of the spectrum). This is called red shift. For sources moving towards the observer, the wavelength is shorter (blue shift). Since source velocities are usually much less than c, the approximation equation is used: Δf/f = Δλ/λ ≈ v/c, where v is the recession speed and c is the speed of light. Red shift of spectral lines from distant galaxies is key evidence for the expansion of the universe.
Applications of the Doppler Effect
The Doppler effect has many practical applications. In medicine, Doppler ultrasound measures the speed of blood flow by detecting the frequency shift of ultrasound waves reflected from moving blood cells. Police radar guns fire microwave pulses and measure the Doppler shift of the reflected signal to find the speed of a car. Astronomers use Doppler shifts in stellar spectra to determine whether a star is moving towards or away from Earth and at what speed. The effect also explains why galaxies are receding from us and forms part of the evidence for the Big Bang.
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Frequently Asked Questions, IB Physics Doppler Effect
What is Doppler Effect in IB Physics? ↓
The Doppler effect for sound and electromagnetic waves, red shift, applications in astronomy and medicine. HL adds solving problems for source/observer velocity. This topic is part of Theme C (Wave Behaviour) in the current IB Physics syllabus.
Is Doppler Effect SL or HL in IB Physics? ↓
Doppler Effect is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Doppler Effect? ↓
The key equations for Doppler Effect are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Doppler Effect? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Doppler Effect for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.