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Galactic Red-shift (Edexcel IGCSE Physics)
Revision Note
Galactic redshift
- The Doppler effect affects all types of waves, including light
- Light emitted from stars and galaxies will be at a certain wavelength in the visible part of the electromagnetic 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
- The redshift definition is therefore:
The phenomenon of the wavelength of light appearing to increase when the source moves away from an observer
- 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
Light from a star that is moving towards an observer will show blueshift and light from a star moving away from an observer will show redshift
- An increase in wavelength (redshift) is a decrease in frequency and vice versa
The observer in front observes a blue shift, the observer behind observes a redshift
Examiner Tip
You need to know that in the visible light spectrum red light has the longest wavelength and the smallest frequency compared to blue light which has a shorter wavelength and higher 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 away from you. As the motorbike travels 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. Don't get caught up on the redshift definition, but focus on the application of the phenomenon in different contexts.
The expanding Universe
- Galactic redshift provides evidence for the Big Bang Theory and the expansion of the universe
- The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to the Earth from a distant galaxy
Redshift diagram for a light spectrum
Comparing the light spectrum produced from the Sun and a distant galaxy. The spectral lines from the distant galaxy are redshifted.
Red shift provides evidence that the Universe is expanding because:
- Red shift is observed when the spectral lines from the distant galaxy move closer to the red end of the spectrum
- This is because light waves are stretched by the expansion of the universe so the wavelength increases (or frequency decreases)
- This indicates that the galaxies are moving away from us
- Light spectrums produced from distant galaxies are red shifted more than nearby galaxies
- This shows that the greater the distance to the galaxy, the greater the redshift
- This means that the further away a galaxy is, the faster it is moving away from the Earth
- These observations imply that the universe is expanding and therefore support the Big Bang Theory
Graph showing the greater the distance to a galaxy, the greater the redshift
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