Evaluate — 5 marks
Astronomers observe light from a distant galaxy and detect that the wavelength of a specific hydrogen spectral line has shifted from 656 nm (its rest wavelength) to 750 nm. The astronomers use this observation to determine information about the galaxy's motion relative to Earth.
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(a) Calculate the wavelength shift for the hydrogen spectral line observed from the distant galaxy.
[1 mark]
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(b) State what the observation of a red-shift tells us about the motion of the distant galaxy relative to Earth.
[1 mark]
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(c) Evaluate whether the magnitude of the red-shift observed in this galaxy provides reliable evidence for the expansion of the universe. In your answer, consider what other factors might affect the observed wavelength of light from distant galaxies.
[3 marks]
Show mark scheme
- (a) Wavelength shift = 750 - 656 = 94 nm (or 9.4 × 10⁻⁸ m)
- (b) The galaxy is moving away from Earth / receding from Earth
- (c) Red-shift is consistent with the expansion of the universe as galaxies moving away produce longer wavelengths
- (c) However, a single observation of one galaxy is not sufficient as it could be affected by local motion/peculiar velocity relative to the cosmic expansion
- (c) Gravitational fields along the line of sight could cause gravitational red-shift, affecting the observed wavelength independently of the galaxy's recession velocity
Evaluate — 4 marks
Astronomers observe light from a distant galaxy. The hydrogen-alpha spectral line, which has a rest wavelength of 656 nm in the laboratory, is observed at 680 nm in the light from this galaxy. The astronomers use this observation to determine that the galaxy is moving away from Earth.
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(a) Calculate the wavelength shift (Δλ) for the hydrogen-alpha spectral line.
[1 mark]
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(b) The observed red-shift provides evidence that the universe is expanding. Evaluate whether this observation alone is sufficient to conclude that the galaxy is receding from Earth. In your answer, consider what other information or observations would strengthen this conclusion.
[3 marks]
Show mark scheme
- (a) Δλ = 680 - 656 = 24 nm (or 24 × 10⁻⁹ m)
- (b) The observation shows a red-shift (longer wavelength), which indicates the galaxy is moving away according to the Doppler effect
- (b) A single observation of one galaxy provides limited evidence; multiple galaxies at different distances should be observed to establish a pattern consistent with universal expansion
- (b) The relationship between red-shift and distance (Hubble's Law) would provide stronger evidence that recession is systematic rather than random motion, or comparison with recession velocities calculated from red-shift would support the conclusion
Suggest — 3 marks
Astronomers observe light from a distant galaxy using a ground-based telescope. The hydrogen alpha spectral line, which normally has a wavelength of 656 nm in the laboratory, is observed at a wavelength of 680 nm. The galaxy is moving away from Earth.
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(a) Suggest why the observed wavelength of the hydrogen alpha line is longer than its laboratory wavelength.
[1 mark]
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(b) Suggest two reasons why the wavelength shift observed from this galaxy might not accurately represent the universe's expansion.
[2 marks]
Show mark scheme
- (a) The galaxy is moving away from Earth / receding (Doppler effect causes wavelength increase / red-shift)
- (b) The galaxy's own local motion / peculiar velocity may have a component away from Earth in addition to cosmological recession
- (b) Dust or other matter between Earth and the galaxy may absorb shorter wavelengths preferentially, making the shift appear larger than it actually is / interstellar reddening
Explain — 5 marks
Astronomers observing a distant galaxy using a powerful telescope notice that the light received from this galaxy shows a shift towards longer wavelengths (red end of the spectrum) compared to the expected wavelengths of known elements. This observation is consistent across multiple spectral lines from hydrogen and helium in the galaxy's light.
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(a) Explain why the light from this distant galaxy appears shifted towards the red end of the spectrum.
[2 marks]
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(b) Explain how observations of red-shift from many distant galaxies provide evidence for the Big Bang theory and an expanding universe.
[3 marks]
Show mark scheme
- (a) The galaxy is moving away from Earth/moving at high speed away
- (a) This causes the wavelength of light to increase/frequency to decrease (Doppler effect)
- (b) Distant galaxies show red-shift, indicating they are all moving away from us
- (b) The further away the galaxy, the greater the red-shift/the faster it is receding
- (b) This shows space is expanding/the universe is expanding; all galaxies are moving apart from each other, consistent with the Big Bang theory where the universe originated from a single point and has been expanding ever since
Show — 2 marks
Astronomers observing distant galaxies have detected that the light emitted by these galaxies is shifted towards the red end of the electromagnetic spectrum. This phenomenon, called red-shift, is observed in the spectral lines of elements such as hydrogen and helium that are known to emit light at specific wavelengths when studied in laboratories on Earth.
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(a) Show that a red-shift in the light from a distant galaxy indicates that the galaxy is moving away from Earth.
[1 mark]
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(b) Show how observations of red-shift from all distant galaxies provide evidence for an expanding universe and support the Big Bang theory.
[1 mark]
Show mark scheme
- (a) Red-shift means observed wavelength is longer than the emitted wavelength / frequency is lower than the emitted frequency
- (a) This is consistent with the Doppler effect / motion away from observer causes wavelengths to increase
- (b) If all distant galaxies show red-shift, they are all moving away from Earth in all directions
- (b) This is consistent with space itself expanding uniformly / the universe is expanding
- (b) This supports the Big Bang theory because the universe must have started from a single point and has been expanding ever since
GCSE Perfect