PHY C14: Doppler Effect

Today we’re covering:

  • Doppler Effect
  • Equation for Doppler Effect

Let’s go!

What is the Doppler Effect?
When a source of waves moves relative to a stationary observer, there is a change in observed frequency.

For example:
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).

Why?
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 sourcefo = fs
Waves travel at a constant speed at a constant frequency in all directions.
Source moves towards observerfo > 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 observerundefinedfo < 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.

Proof:
https://www.khanacademy.org/science/physics/mechanical-waves-and-sound/doppler-effect/v/doppler-effect-formula-for-observed-frequency

Extra Links:

⇐ Previous in Physics: Transverse & Longitudinal Waves
⇒ Next in Physics: Electromagnetic Radiation

One thought on “PHY C14: Doppler Effect

  1. Pingback: 14. WAVES – ProDuckThieves

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