The Doppler Effect: Frequency Shift Due to Relative Motion
- consider standing by a road as a car speeds past.
- The sound of the engine is high-pitched as it approaches, but drops suddenly as it moves away.
- This change in pitch is a classic example of the Doppler effect.
Definition
Doppler effect
The Doppler effect is the change in the observed frequency of a wave when there is relative motion between the source and the observer
The Doppler effect applies to all types of waves, including sound, light, and water waves.
Why Does the Frequency Change?
To understand the Doppler effect, we need to consider two scenarios:
- Stationary Source and Moving Observer
- Moving Source and Stationary Observer
Stationary Source and Moving Observer
- When a source is stationary, it emits wavefronts that spread out evenly in all directions.
- If the observer is moving towards the source, they encounter wavefronts more frequently, resulting in a higher observed frequency.
- Conversely, if the observer is moving away, they encounter wavefronts less frequently, leading to a lower observed frequency.
The wavelength remains unchanged in this scenario because the source is stationary.
Moving Source and Stationary Observer
- When the source is moving, the wavefronts are compressed in the direction of motion and stretched in the opposite direction.
- This causes the wavelength to decrease in front of the source and increase behind it.
- Consider a car moving towards an observer while emitting sound waves.
- The wavefronts in front of the car are closer together, leading to a higher frequency for the observer.
- Behind the car, the wavefronts are spread out, resulting in a lower frequency.
Wavefront Representation: Visualizing the Doppler Effect
Stationary Source and Moving Observer
- In this scenario, the wavefronts are concentric circles centered around the stationary source.
- If the observer moves towards the source, they encounter wavefronts more frequently, resulting in a higher observed frequency.
- If the observer moves away, they encounter wavefronts less frequently, leading to a lower observed frequency.
- Think of wavefronts as runners on a track.
- If you run towards them, you meet them more often.
- If you run away, you meet them less frequently.
