Redshift, Doppler effect, Hubble law and the age of the universe

Section in Chapter 5 in the AQA AS Physics B textbook.

Covers Redshift, Doppler effect, Hubble law, and calculating the age of the universe in more detail than in the textbook.

I made this for peer teaching, useful for a better understanding of the definitions, understanding where the three topics link and where all those equations come from (which they don't explain in the book!?)

Pgs 80-82 in the AQA physics B: physics in context text book

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The red shift
In the simplest terms, 'red shift' is a term used to describe the
fact that the light seen from distant galaxies shows up a little
differently than it does here on Earth.
Each element has a 'fingerprint' in light. This is how we know
which elements are in which stars. There is a certain pattern of
lines associated with each element which identifies it.
However, as we get further and
further out in space, these identifying
lines, while keeping the same
identifying patterns for each element,
appear shifted somewhat to the red
end of the spectrum -- thus causing
the light to appear redder than it
would be here on earth.…read more

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Official definition of red shift
The redshift is an astronomical term that describes
the shifting of the spectral lines of atoms towards
the red end of the spectrum when compared with a
laboratory standard here on earth. Consequently,
the redshift, z, is defined as the measured change in
wavelength, when compared with the standard,
divided by that laboratory standard wavelength. If
the change in wavelength is given by and the
laboratory standard wavelength is represented by ,
then the redshift is defined as z = /
[Couderc, 1960 p.10, 91;
Audouze & Israel, 1985 p.382,
356]…read more

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Example of red shift
The element is moving
closer to you, therefore
the element spectra is
shifted towards the blue
end of the
Element is spectrum
at rest
The element is moving
further away from you,
therefore the element
spectra is shifted towards
the red end of the
spectrum.
So, to sum up the example above:
Element that is cLoser ­ bLue shift
Element that is fuRther away ­
Red shift…read more

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The Doppler effect
The Doppler effect is easily observed in the case of sound
waves. The sound from the siren on an ambulance changes from
high pitch as it approaches a listener to a lower pitch as it moves
away. The true pitch is heard as the ambulance passes by.
The effect is observed whenever
there is relative motion between
the source and the listener. It
does not matter whether one or
both are moving as long as they
are moving closer together or
further apart. In astronomy it is
the motion of the source relative
to us on Earth that is important.…read more

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Example of the Doppler effect
As the source moves the waves are being bunched into a smaller
distance in the direction in which it is moving and stretched in the
opposite direction. This means that the wave fronts pass an
observer ahead of the source more often so that the sound has a
higher frequency.
behind the source ahead of the source
STRETCHED BUNCHED
S
A B C
W1 W2 W2 W1
Wave fronts marked W1 emitted when S was
at A
Wave fronts marked W2 emitted when S was
at B…read more

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