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Galactic Redshift (CIE IGCSE Physics)
Revision Note
Galactic redshift
- When a stationary object emits waves, they spread out symmetrically
- If the source of the waves moves, the waves can become squashed together or spread out
- This change in wavelength is known as the Doppler effect
- This effect applies to both sound waves and electromagnetic radiation (i.e. light waves)
Doppler effect of light
- If a source of light moves towards an observer, the observed wavelength decreases
- The wavelength of light shifts towards the blue end of the spectrum
- This is called blueshift
- If a source of light moves away from an observer, the observed wavelength increases
- The wavelength of light shifts towards the red end of the spectrum
- This is called redshift
Redshift and blueshift of light
When a light source moves away from an observer, redshift is observed, and when a light source moves towards an observer, blueshift is observed
Observing redshift in distant galaxies
- When astronomers compare light from glowing hydrogen in distant galaxies with light from glowing hydrogen on Earth, the light appears to be redshifted
- This means the observed wavelength has increased as the light travelled from the galaxy to the Earth
- This shows that distant stars and galaxies are moving away (receding) from the Earth
Redshift of light from a distant galaxy
Light emitted from glowing hydrogen in distant objects appears to be shifted towards the red end of the spectrum, showing they are moving away from Earth
- The greater the observed redshift:
- the greater the distance to a galaxy
- the faster the galaxy is moving away from Earth
Exam Tip
You need to know that in the visible light spectrum, red light has the longest wavelength and the smallest frequency.
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