# What is the Doppler shift, and why is it important to astronomers?

Jul 7, 2015

The Doppler effect is caused due to a change in frequency (and wavelength) of light when there is a relative motion between the source and the observer

#### Explanation:

In classical physics, when the speeds of source and the receiver relative to the medium are lower than the velocity of waves in the medium, the relationship between observed frequency $f$ and emitted frequency ${f}_{0}$ is

where $c$ is velocity of waves in the medium;
${v}_{r}$ is velocity of the receiver relative to the medium; positive if the receiver is moving towards the source (and negative in the other direction);
${v}_{s}$ is velocity of the source relative to the medium; positive if the source is moving away from the receiver (and negative in the other direction).

• Use in Astronomy * :

• The Doppler effect results in something known as the red and blue shift..
Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blueshift and those of a receding astronomical light source exhibit a redshift.

• It is also used to measure the speed of stars and galaxies.
Aug 5, 2015