THE DOPPLER EFFECT
4 marks · NormalWhen astronomers observe a distant galaxy through a telescope, they analyse its light spectrum to investigate its movement.
State what is meant by red-shift in the context of light from galaxies.
Explain what the observation of red-shifted light tells astronomers about the motion of distant galaxies.
Describe how the size of the observed red-shift relates to the distance of a galaxy from Earth.
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(a) The observed increase in wavelength (or decrease in frequency) of light from a galaxy, causing it to appear more red than expected.
(b) If light from a galaxy is red-shifted, it means the galaxy is moving away from the observer (Earth). The shift provides evidence that distant galaxies are receding, implying the universe is expanding.
(c) The greater the red-shift, the further away the galaxy is from Earth (or, larger red-shift means greater distance).
Common mistakes: Omitting link between red-shift and recessional motion; Incorrectly describing blue-shift instead of red-shift
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Marking points
B1 (a) · 1 mark · Correct definition of red-shift as increase in wavelength/decrease in frequency; must refer to spectrum/light from galaxies.
B1 (b) · 1 mark · States that red-shifted galaxies are moving away from Earth or observer.
B1 (b) · 1 mark · Mentions that this means the universe is expanding (or that galaxies are receding).
B1 (c) · 1 mark · States that a larger red-shift corresponds to greater distance from Earth or greater velocity of recession.
Worked solution
a) Red-shift is the observed increase in wavelength (or decrease in frequency) of light from a galaxy, causing the light to appear more red than expected.
b) If the light from a galaxy is red-shifted, it means the galaxy is moving away from the observer (Earth). The observation that most galaxies are red-shifted suggests that they are all moving away from us, which provides evidence that the universe itself is expanding.
c) The greater the red-shift, the greater the distance of the galaxy from Earth (or, larger red-shift means the galaxy is further away).