# The Doppler Effect

Christian Johann Doppler, born November 29, 1803 in Salzburg, Austria died
March 17, 1853 in Venice. Christian Doppler an Austrian Physicist who first described
how the detected frequency of light and sound waves is affected by the relative motion of
the source and the detector. This aspect became known as the Doppler Effect. Christian
was educated at the Polytechnical Institute in Vienna. Doppler became director of the
physical Institute and professor of experimental physics of the University of Vienna in
1850. In 1842 he published Uber das farbige Licht der Dopperlsterne which means
Concerning the Coloured Light of Double Stars, which contained his first statement of
the Doppler effect. He then theorized that since the pitch of sound from a moving source
varies for a stationary observer, the colour of the light from a star should alter, according
to the star?s velocity relative to Earth.
The cause of the Doppler effect isn?t just sound. There is also light. As the
source of the wave approaches or moves away to a person. For example you can tell if a
light is far away, or close to you without really judging where the light is at. All you have
to do is if a light is far away from you then the light is small, and continues to get smaller
as it moves away. And if the light is moving towards you then the light becomes bigger,
and brighter. Another example of this is if you ever drove in a car at night you can see a
car from far away but the headlights of the other car look like they are more seperated
apart. And as the vehicle moves closer to you, the lights seem that they are getting closer
together.
What happens to a pitch or frequency when the source is moving towards the
person or away from him/her? It has been established from sound waves that the
frequency is higher when a sound source is moving toward the person and lower when it
is moving away from him/her. Another example of the Doppler effect is the way a police
car?s siren changes in pitch when it passes by. The sound waves in front of the car are
condensed, while the trailing sound waves are longer and of a lower pitch. Light from an
object moving towards the Earth is shifted toward the blue end of the spectrum, where as
light from objects moving away is shifted toward the red end.
The human ear cannot hear all possible frequencies. Very few people can hear
any fewer than 16 Hz or any more than about 20 kHz (kilohertz?1 kHz equals 1,000 Hz).
Music rarely makes use of this whole range of audible frequencies. The lowest note on a
piano has a frequency of 27 Hz and the highest note a little more than 4 kHz. Frequency-
modulation (FM) radio stations broadcast notes up to 15 kHz. These can be heard through
The intensity of a sound has nothing to do with its pitch. A high tone can be either
loud or soft, and so can a low tone. Intensity depends upon the strength, or amplitude, of
the vibrations producing the sound. A piano string, for example, vibrates gently if the key
is struck softly. The string swings back and forth in a narrow arc, and the tone it sends
out is soft. If the key is struck forcefully, however, the string swings back and forth in a
wider arc. The stronger vibration then produces a louder tone.
Redshift is a Doppler effect which states that if a galaxy is moving away, the
spectral line of that galaxy observed will have a shift to the red end. The faster the galaxy
moves, the more shift it has. If the galaxy is moving closer, the spectral line will show a
blue shift. If the galaxy is not moving, there is no shift at all. However, as astronomers
observed, the more distance a galaxy is located from Earth, the more redshift it shows on
the spectrum. This means the further a galaxy is, the faster it moves.

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