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 f_s.
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 f_s.
When these emitted waves reach an observer, the observers detect a frequency of f_o.
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 – v_s.
•  If the wave source is moving AWAY from the observer, use v + v_s.

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

Extra Links:

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