The Doppler effect
In our daily life everybody experiences the Doppler effect from time to time. When a police car approaches us with shrieking siren,
then
we hear the sound of the siren with a higher pitch than when the car is
standing still. As soon as the car passes the pitch changes and we hear a
much lower sound. This is called the Doppler effect.
<Wavelength and frequency.
The pitch of the sound is determined by the wavelength
of the sound that is propagated, or the frequency
which is the number of vibrations per second. The shorter the wavelength,
the more vibrations, the higher the sound.
When the police car is approaching you the sound waves move towards you
and so they are compressed. The frequency becomes higher and we hear
the sound with a higher pitch than the real sound of the siren.
When the car passes, the sound waves are stretched because the sound source
is moving away from us, the frequency gets lower and so we hear a lower
pitch.
Color shift.
Also the electromagnetic
radiation is affected, which is the same as the light emitted by stars. The frequency
of this radiation determines the color we see. The higher the frequency,
the more blue we can see in the color; when the frequency is lower
the color turns more red.
The color of a star that is moving fast towards us or away from us is observed
in other colors than its real color. When the star is moving away we observe
a more redish color, the color has shifted towards the red part of
the spectrum.
The more red the color, the faster the star is moving
away from us. The same applies for the blue-shift when
the star approaches us.
This spiral galaxy is rotating, so we see the lower part approaching (blue) and the upper moving away from us (red). From this the rotation speed can be determined. |
Color shift as a speed indicator.
The color shift is a measure for the speed of motion of stars. By measuring the colors of stars and comparing them with each other we can determine the various speeds of them. The Doppler effect is applicable to all types of radiation, also to very short wavelengths such as röntgen radiation, radio waves etc. which we normally cannot see. The satellite ROSAT has made a map of the Universe by means of röntgen radiation. |