Galactic Red-shift (AQA GCSE Physics)
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Galactic Red-shift
Usually, when an object emits waves, the wavefronts spread out symmetrically
If the wave source moves, the waves can become squashed together or stretched out
Diagram showing the wavefronts produced from a stationary object and a moving object
A moving object will cause the wavelength, λ, (and frequency) of the waves to change:
The wavelength of the waves in front of the source decreases (λ – Δλ) and the frequency increases
The wavelength behind the source increases (λ + Δλ) and the frequency decreases
This effect is known as the Doppler effect
The Doppler effect also affects light
If an object moves towards an observer the wavelength of light decreases
This is known as blueshift as the light moves towards the blue end of the spectrum
If an object moves away from an observer the wavelength of light increases
This is known as redshift as the light moves towards the red end of the spectrum
Light from a star that is moving towards an observer will be blueshifted and light from a star moving away from an observer will be redshifted
The observer in front observes a blue shift, the observer behind observes a red shift
Examiner Tips and Tricks
You need to know that in the visible light spectrum red light has the longest wavelength and the smallest frequency.
To help you to remember what happens to the wavelength and the frequency of an object as it moves further away, it is useful to think about how the sound of a motorbike would change as it travels past and then away from you. As the motorbike moves away from you the pitch of the sound will become lower. This means the frequency of the sound is decreasing. If the frequency has decreased, the wavelength must also have increased.
The Expanding Universe
The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to us from a distant galaxy
Comparing the light spectrum produced from the Sun and a distant galaxy
The diagram also shows that the light coming to us from distant galaxies is redshifted
The lines on the spectrum are shifted towards the red end
This indicates that the galaxies are moving away from us
If the galaxies are moving away from us it means that the universe is expanding
The observation of redshift from distant galaxies supports the Big Bang theory
Another observation from looking at the light spectrums produced from distant galaxies is that the greater the distance to the galaxy, the greater the redshift
This means that the further away a galaxy, the faster it is moving away from us
Graph showing the greater the distance to a galaxy, the greater the redshift
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