COSMIC MICROWAVE BACKGROUND (CMB) RADIATION
4 marks · NormalA scientist studies the light emitted from a nearby galaxy and observes that absorption lines have shifted compared to those from our Sun.
What is this phenomenon, and what does it tell us about the motion of the galaxy relative to Earth?
Explain why the absorption lines move towards the red end of the spectrum.
Explain why the absorption lines move towards the red end of the spectrum.
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(a) Red-shift; the galaxy is moving away from Earth.
(b) Light waves are stretched as the galaxy moves away, increasing their wavelength (shift towards red).
Common mistakes: stating blue-shift for moving away; saying frequency increases for red-shift
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Marking points
B1 (a) · 1 mark · Correctly states red-shift.
B2 (a) · 1 mark · Recognises galaxy is moving away from Earth.
B3 (b) · 1 mark · States light waves are stretched or wavelength increases.
B4 (b) · 1 mark · Links stretched light to the shift towards the red end of the spectrum.
Worked solution
a) This phenomenon is red-shift. It tells us that the galaxy is moving away from the Earth.
b) The absorption lines move towards the red end of the spectrum because, as the galaxy moves away, the light waves it emits are stretched out, which increases their wavelength. Increased wavelength means the lines shift to the red end.