Today we’re covering:
- Doppler Effect
- Equation for Doppler Effect
What is the Doppler Effect?
When a source of waves moves relative to a stationary observer, there is a change in observed frequency.
A car emits a siren.
When the car is stationary, you hear a sound of frequency fs.
When the car heads towards you, you hear a HIGHER frequency (pitch).
When the car passes & drives away from you, hear a LOWER frequency (pitch).
Wave sources emit waves at a constant SOURCE FREQUENCY.
We’ll call this fs.
When these emitted waves reach an observer, the observers detect a frequency of fo.
This observed frequency depends on the RELATIVE MOTION between the source & the observer:
|Stationary source||fo = fs|
Waves travel at a constant speed at a constant frequency in all directions.
|Source moves towards observer||fo > fs|
Wavefronts in “front” of the moving source are compressed closer together. Observer detects more wavefronts per second = higher frequency is observed.
|Source moves away from observer||fo < fs|
Wavefronts “behind” the moving source are further apart. Observer detects less wavefronts per second = lower frequency is observed.
What equation describes the Doppler Effect?
- If the wave source is moving TOWARDS the observer, use v – vs.
- If the wave source is moving AWAY from the observer, use v + vs.