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IGCSE · 6 · multiple choice

Space physics · Question 6

Space physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
demanding
Marks
1
Topics
1
Answer
Complete
The same spectral line from a laboratory source and from a distant galaxyspectral linespectrum from alaboratory sourcespectrum fromthe galaxyincrease in wavelengthwavelength
Fig. 6.1Two spectra drawn one above the other against a common horizontal wavelength scale that increases to the right. The upper strip is the spectrum of a source in the laboratory on Earth, with one spectral line marked in it. The lower strip is the spectrum of light received from the distant galaxy, in which the same line appears further to the right. A dashed line dropped from each line marks its position on the scale, and the gap between the two positions is labelled as the increase in wavelength.
multiple choice1 mark

Light from a distant galaxy shows a spectral line at a wavelength longer than the wavelength of the same line measured in a laboratory on Earth. What does this show?

  1. A

    The galaxy is moving towards the Earth.

  2. B

    The galaxy is moving away from the Earth.

  3. C

    The galaxy is hotter than the laboratory source.

  4. D

    Light slows down as it travels across space.

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Answer overviewCorrect answer: B — The galaxy is moving away from the Earth.
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is B: The galaxy is moving away from the Earth.

  2. 2

    A is the answer of a candidate who knows the term redshift but not which way it runs. D contradicts the constancy of the speed of light in a vacuum, on which the whole method depends.

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