### Doppler Shift of Sound

- The whistle of a train or the siren of an ambulance appears to increase in frequency (sounds higher in pitch) as it moves away from you
- This frequency change due to the relative motion between a source of sound or light and an observer is known as the
**doppler effect**(or**doppler shift**) - When the observer (e.g. yourself) and the source of sound (e.g. ambulance siren) are both
**stationary**, the waves are at the**same**frequency for both the observer and the source

*Stationary source and observer*

- When the source starts to move
**towards**the observer, the wavelength of the waves is**shortened**. The sound therefore appears at a**higher**frequency to the observer

*Moving source and stationary observer *

* *

- Notice how the waves are closer together between the source and the observer compared to point P and the source
- This also works if the source is moving away from the observer. If the observer was at point P instead, they would hear the sound at a lower frequency due to the wavelength of the waves
**broadening**

** **

- The frequency is
**increased**when the source is moving**towards**the observer - The frequency is
**decreased**when the source is moving**away**from the observer

### Calculating Doppler Shift

- When a source of sound waves moves relative to a stationary observer, the observed frequency can be calculated using the equation below:

*Doppler shift equation *

- The wave velocity for sound waves is 340 ms
^{-1} - The ± depends on whether the source is moving towards or away from the observer
- If the source is moving
**towards**, the denominator is v – v_{s} - If the source is moving
**away**, the denominator is v + v_{s}

- If the source is moving

* *

#### Exam Tip

Be careful as to which frequency and velocity you use in the equation. The ‘source’ is always the object is moving and the ‘observer’ is always stationary.

### The Doppler Effect

- The doppler shift is observed by all waves including sound and light
- A frequency change due to the relative motion between a source of sound or light and an observer is known as the
**doppler effect**(or**doppler shift**) - When the observer (e.g. yourself) and the source of sound (e.g. ambulance siren) are both
**stationary**, the waves are at the**same**frequency for both the observer and the source - When the source starts to move
**towards**the observer, the wavelength of the waves is**shortened**- For sound waves, sound therefore appears at a
**higher**frequency to the observer - For light waves, the light shifts towards
**blue**due to its higher frequency

- For sound waves, sound therefore appears at a

- When the source starts to move
**away**the observer, the wavelength of the waves is**broadens**- For sound waves, sound therefore appears at a
**lower**frequency to the observer - For light waves, the light shifts towards
**red**due to its lower frequency

- For sound waves, sound therefore appears at a
- For light waves, remember that red light has a longer wavelength than blue light
- Red and blue shift is observed in spectral lines of planets and is used to find the radius of the planets orbit around distant stars