Cosmology
4 marks · EasyLight from the Sun has a wavelength of 500 nm. Light observed from a distant galaxy shows the same absorption line at a wavelength of 560 nm.
Calculate the change in wavelength for the absorption line.
The speed of light is m/s. Using the Doppler effect equation , calculate the speed at which the galaxy is moving away from Earth. Give your answer in km/s.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 60 nm
(b) The speed is 36,000 km/s
Common mistakes: Using incorrect units (not converting to km/s); Swapping reference and observed wavelength
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correctly calculates change in wavelength (60 nm)
M1 (b) · 1 mark · Substitutes correct values into Doppler equation (follow-through)
M2 (b) · 1 mark · Converts units where necessary (to km/s) (follow-through)
A2 (b) · 1 mark · Final answer for galaxy speed (36,000 km/s) (follow-through)
Worked solution
(a)
(b)
= 0.12 × 3.0 × 10^8
= 0.36 × 10^8 m/s
= 3.6 × 10^7 m/s
To km/s: 3.6 × 10^7 m/s ÷ 1000 = 36,000 km/s