Doppler Effect

00:00 7 marks

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A stationary sound source emits waves of frequency 440 Hz. An observer moves towards the source at a constant speed of 20.0 m s-1. The speed of sound in air is 340 m s-1.

Part a

3 minutes 2 marks

Question

Calculate the frequency detected by the observer. [2]



Hint Hint

Hint

Use the Doppler effect formula for an observer moving towards a stationary source. The observed frequency should be higher than the source frequency.

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466 Hz

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Part b

2 minutes 1 marks

Question

The observer now moves away from the stationary source with the same speed. Calculate the new frequency detected by the observer. [1]



Hint Hint

Hint

Use the Doppler effect formula for an observer moving away from a stationary source. The observed frequency should be lower than the source frequency.

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Complete Answer

414 Hz

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Part c

4 minutes 3 marks

Question

Explain why the Doppler effect equations differ for sound waves and electromagnetic waves when considering the motion of the source versus the motion of the observer. [3]



Hint Hint

Hint

Consider the medium of propagation for sound versus EM waves and the implications of special relativity for light.

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Sound requires a medium for propagation, while electromagnetic waves do not. For sound, the velocities of the source (\\(u_s\\)) and observer (\\(u_o\\)) are measured relative to the medium, leading to different formulas depending on who is moving (e.g., \\(f’ = f(v/(v\\pm u_s))\\) vs \\(f’ = f((v\\pm u_o)/v)\\)). For light (in vacuum), the speed of light \\(c\\) is constant for all inertial observers, so only the *relative* velocity between the source and observer matters (relativistic Doppler effect).

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Part d

1 minutes 1 marks

Question

State one practical application of the Doppler effect. [1]



Hint Hint

Hint

Think about technologies that measure speed or motion using waves.

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Radar (speed guns) / Medical ultrasound (blood flow) / Astronomy (redshift/blueshift of stars/galaxies)

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