AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
There is an observed increase in the wavelength of light from most distant…
Full specification wording
Red-shift (physics only): There is an observed increase in the wavelength of light from most distant galaxies.
Light from a distant galaxy arrives with a longer wavelength than the same light would have if the galaxy were at rest, and the effect is measured using the dark absorption lines in the galaxy's…
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Light from a distant galaxy arrives with a longer wavelength than the same light would have if the galaxy were at rest, and the effect is measured using the dark absorption lines in the galaxy's spectrum. Those lines are produced by known elements and appear at known wavelengths in the laboratory, so when the whole pattern is found shifted towards the red end of the visible spectrum, the size of the shift can be measured precisely. A longer wavelength means a lower frequency, and since red is the longest wavelength of visible light the effect is called red-shift. It is observed in almost all galaxies, which tells us they are moving away from us; a few nearby galaxies are blue-shifted instead, because they are approaching.
What earns the marks
- An observed increase in the wavelength of the light.
- The light coming from galaxies that are moving away from us.
- The shift being measured from the positions of dark lines in the spectrum.
- A longer wavelength corresponding to a lower frequency.
Common mistakes and how to put them right
Writing only that the light waves are stretched, which the mark scheme ignores.
Saying the light changes colour to red, when the shift is measured right across the spectrum.
Describing a decrease in wavelength, which would be a blue-shift.
Saying the galaxies look redder because they are hot, rather than because they are receding.
Exam tips
- Use the words observed increase in wavelength, because that is the phrasing that earns the mark.
- Mention the dark lines when a spectrum is shown, and say which way the pattern has moved.
- Say that the galaxy is moving away from us, not simply that it is moving.
Try the lesson
The start of the Red-shift (physics only): There is an observed increase in the wavelength of light from most distant galaxies. lesson, made from checked teaching notes for this specification point.
Have a go first
What name is given to the observed increase in the wavelength of light from most distant galaxies?
Listen
Light from a distant galaxy arrives with a longer wavelength than the same light would have if the galaxy were at rest, and the effect is measured using the dark absorption lines in the galaxy's spectrum. Those lines are produced by known elements and appear at known wavelengths in the laboratory, so when the whole pattern is found shifted towards the red end of the visible spectrum, the size of the shift can be measured precisely. A longer wavelength means a lower frequency, and since red is the longest wavelength of visible light the effect is called red-shift. It is observed in almost all galaxies, which tells us they are moving away from us; a few nearby galaxies are blue-shifted instead, because they are approaching. A strong answer earns marks for these points. an observed increase in the wavelength of the light. the light coming from galaxies that are moving away from us. the shift being measured from the positions of dark lines in the spectrum. a longer wavelength corresponding to a lower frequency. Exam tip: Use the words observed increase in wavelength, because that is the phrasing that earns the mark.
Key words
Say what each word means before you turn the card.
red-shift
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increase in wavelength
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absorption line spectra
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receding galaxies
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See every definition
- red-shift
- An observed increase in the wavelength of light from most distant galaxies, called red-shift because red is the longest wavelength of visible light.
- increase in wavelength
- The observed change in light from most distant galaxies, where the light arrives with a longer wavelength than the same light would have if the galaxy were at rest.
- absorption line spectra
- Dark lines in a galaxy's spectrum, produced by known elements and appearing at known wavelengths in the laboratory, used to measure the size of the shift.
- receding galaxies
- Galaxies moving away from us, shown by red-shift observed in almost all galaxies, while a few nearby galaxies are blue-shifted because they are approaching.
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