# 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:

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.

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