Doppler Effect Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Doppler Effect.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

The change in observed frequency of a wave that occurs when the source and the observer are moving relative to each other.

An ambulance siren sounds higher-pitched approaching, lower-pitched receding.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Motion compresses or stretches the waves, changing perceived frequency.

Common stuck point: The actual frequency doesn't change—only the observed frequency does.

Worked Examples

Example 1

easy
An ambulance siren emits sound at 700 \text{ Hz}. As the ambulance approaches you at 30 \text{ m/s}, what frequency do you hear? Use v_{\text{sound}} = 340 \text{ m/s}.

Solution

  1. 1
    Doppler effect for approaching source: f' = f \times \frac{v}{v - v_s}.
  2. 2
    f' = 700 \times \frac{340}{340 - 30} = 700 \times \frac{340}{310}
  3. 3
    f' = 700 \times 1.097 \approx 768 \text{ Hz}

Answer

f' \approx 768 \text{ Hz}
The Doppler effect causes the perceived frequency to increase when a sound source approaches and decrease when it moves away. This is why an ambulance siren sounds higher-pitched as it approaches.

Example 2

medium
A train whistle blows at 500 \text{ Hz}. You are stationary and hear 475 \text{ Hz}. Is the train approaching or moving away? What is the train's speed? Use v_{\text{sound}} = 340 \text{ m/s}.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
A car horn emits 400 \text{ Hz}. You drive toward the stationary car at 20 \text{ m/s}. What frequency do you hear? Use v_{\text{sound}} = 340 \text{ m/s}.

Example 2

hard
Police radar emits microwaves at 10.5 \text{ GHz}. The reflected signal from an approaching car has a frequency shift of 3500 \text{ Hz}. What is the car's speed? Use c = 3 \times 10^8 \text{ m/s} and the Doppler formula \Delta f = \frac{2v_s f}{c} for electromagnetic waves.
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Related Concepts

FrequencyWave Speed

Background Knowledge

These ideas may be useful before you work through the harder examples.

frequencywave speed