Cosmology: 2.
Explaining the Doppler effect
of light with reference to cosmology models of the universe
Doc Brown's Physics exam study revision notes
There are various sections to work through, after 1 they can be read and studied in any order.
INDEX for physics notes on
COSMOLOGY
(2) Explaining the Doppler effect
with reference to cosmology and models of the universe
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Know that current evidence suggests that
the universe is expanding and that matter and space expanded violently and
rapidly from a very small initial ‘point’, ie the universe began with a ‘big
bang’.
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The universe is big, but seems to be
getting bigger and all the galaxies seem to be moving away from each other -
so how are we going to explain this cosmological expansion?
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First, a bit of classical physics
to help you understand this page on the
red-shift and expanding universe.
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If a wave source is moving
relative to an observer there will be a change in the observed wavelength
and frequency. This is called the Doppler effect.
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To observe the Doppler effect, the wave source could be
sound, visible light, microwave or any other part of the electromagnetic spectrum.
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When the source moves away from
the observer, the observed wavelength increases and the frequency decreases
because the waves get stretched out..
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When the source moves towards
the observer, the observed wavelength decreases and the frequency increases
because the waves become compressed. (diagrams below)
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You experience the Doppler
effect quite clearly when a loud racing car or train passes by you.
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As the loud moving object
approaches you, the frequency (pitch) of the sound waves increases as
oncoming sound waves are compressed closer together (increasingly shorter wavelength).
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As the object moves away from
you, the frequency (pitch) decreases as the sound waves stretch out (wavelength
becoming increasingly longer).
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This is an effect, quite
distinct from the fact that the sound of the moving object becomes louder
then softer as the object passes you.
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Light waves also exhibit the Doppler effect.
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The speed of light is a constant
in outer space, but in the case of a bright object
like a bright galaxy emitting white light however near or far away
...
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(i) if there is no great
difference in speed relative to planet Earth, a white object will look
white,
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(ii) if the object was
hurtling towards us at great speed the light waves would be
compressed, wavelength decreases, frequency increases, the light
shifts towards the blue end of the spectrum (a blue-shift), so
the object would look a little more bluish than it really is,
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(iii) if the object is moving
away from us at great speed the light waves are stretched,
wavelength increases, frequency decreases and the light shifts
towards the red end of the spectrum (red-shift), so the
object would look a little more reddish than it really is.
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The Doppler effect on light
waves is used to measure the speed at which the galaxies seem to be moving
away from us and our galaxy in all directions!
-
This 'astronomical' Doppler
effect (quite literally and metaphorically!) is one of the main pieces of evidence for the 'Big Bang' theory of
the expansion of our universe from some 'point' at 'time zero' around 14
billion years ago!
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This piece of evidence for the big
bang theory is called the red shift
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More details of the analysis of
light from stars using the
science of spectroscopy is explained in
Part
3, and, after you have a learned about spectroscopy of
how elements are detected using emitted light from very hot atoms of stars.
we can then explain the red shift in more detail in
Part 4.
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INDEX for physics notes on
COSMOLOGY
Keywords, phrases and learning objectives for
?
Know and understand that if a wave source is moving
relative to an observer there will be a change in the observed wavelength
and frequency.

Be able to explain the Doppler effect in a
cosmological context of the universe.
You should know that there is an observed increase
in the wavelength of light from most distant galaxies suggesting they are
moving away from us in all directions.
Know that the Doppler shift in frequency and
wavelength of light incoming to Earth from distant objects in the
Universe.
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