Stellar Velocity Page 9

Radial velocity

First about the Doppler effect

We all know the effect of the changing of wavelength and frequency of waves when the source is moving with respect to the observer, but we may not know it under its formal name: the Doppler effect. It is named after Christian Doppler, who formulated this principle in 1842.

It is easy to demonstrate the Doppler effect with sound waves. When the distance between the sound source and the observer is changing, the observer perceives the sound to be at a higher pitch (frequency) when the distance is decreasing, and to be at a lower pitch when the distance is increasing. This is caused by the fact that, while waves travel at a certain speed, due to the relative motion the number of waves that reach the observer per second changes, hence the apparent frequency changes. When a source is approaching we hear a higher frequency and when the source is receding we hear a lower frequency.

Click the Play button to hear the sound


f' in the graph is the apparent frequency; f0 is the nominal frequency.
When the car approaches you hear a higher frequency,
when the car recedes you hear a lower frequency.
Only at the point of closest approach you hear the actual frequency.

Imagine for instance that you are standing on the side of a race track and you hear a race car coming towards you and then passing by. The sound you hear distinctly changes its frequency when the car passes you. When the car approaches, it has a relatively high frequency and when it passes, the pitch becomes lower.

Only at the precise moment that the car is right in front of you (no relative velocity between the car and yourself) you hear the true frequency of the sound of the car. When the car then recedes, you hear a lower frequency. This apparent change in frequency (for waves we can also say wavelength) is due to the so-called Doppler effect.