The Doppler Effect and The Red Shift

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The Doppler Effect and The Red Shift
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Doppler Effect

• This is the apparent change in the frequency of
  waves due to the motion of the source or of the
  observer.

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Doppler Effect

• This is the apparent change in frequency of waves
  due to the motion of the source/observer
• Note The higher the frequency the higher the
  pitch
• e.g. car sounding its horn coming toward you,
  ambulance siren coming toward you etc

An example of this would be a formula one car as
it drives past Note how the frequency of the
sound changes
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Using the Doppler Shift to measure the motion of
stars and galaxies in the Universe
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First, consider how we can disperse light into
its different colours - I.e. how we can form a
spectrum.
Each colour corresponds to a particular
wavelength of light red the longer wavelength,
blue the shorter.
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If you pass the light through a gas something
different is seen
Some wavelengths of light are absorbed by the gas
creating an absorption spectrum.
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If the light source is moving away the absorption
spectra look a little different
Spectrum of stationary cloud
helium
(Gas cloud receding from prism/observer)
Spectrum of receding cloud -gt
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The absorption lines have all been shifted
towards the longer wavelength end (red end)
This is the same effect that occurs for sound
waves it is called the Doppler Effect.
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In this way, the speed of stars and even galaxies
can be measured from their spectra. An
important, early application of this technique
was by Edwin Hubble, who measured the speeds of
distant galaxies and found that, on average, the
more distant a galaxy is from us, the faster it
is moving.
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Red shift summary
This implies that, in the past, the galaxies were
much closer together, and indeed, at some moment
in the past, all the stuff of the Universe was
located at a single point, from which we see the
Universe expanding today. By measuring the speed
of galaxies and their distance from us, we can
easily calculate the time in the past at which
this expansion began - the so-called Big Bang
this is estimated as some 15 billion (15 thousand
million) years ago.