Doppler effect, redshift and the expanding universeEdexcel A-Level Physics: Flashcards

What these 13 flashcards ask

What is the Doppler effect?
What happens to the observed frequency when a source moves towards the observer?
Why does a source moving away from you give a lower frequency?
State the equation for redshift.
When is z ≈ v/c a valid approximation?
What does a redshift tell you about a galaxy?
What is a blueshift?
State the Hubble law.
What is the approximate value of the Hubble constant in SI units?
How does Hubble's law support the expanding universe?
How is the age of the universe estimated from H₀?
Why is the age of the universe uncertain?
Why does dark matter matter for the fate of the universe?

Exam questions on Doppler effect, redshift and the expanding universe

An ambulance travels along a straight road at a steady speed while its siren emits a constant note of frequency 700 Hz, measured by the driver. A pedestrian stands at the side of the road, and the ambulance drives past her.
Explain why the pedestrian hears a higher frequency than 700 Hz as the ambulance approaches.2 marks
Astronomers study the light from a distant galaxy. The hydrogen line that has a wavelength of 656.3 nm when measured in a laboratory is observed in the galaxy's spectrum at a wavelength of 662.9 nm. Take $c = 3.00 \times 10^{8}$ m s⁻¹.
Calculate the speed at which the galaxy is receding from the Earth.2 marks
A galaxy is a distance of $4.5 \times 10^{24}$ m from Earth. Take the Hubble constant as $H_0 = 2.2 \times 10^{-18}$ s⁻¹ and $c = 3.00 \times 10^{8}$ m s⁻¹. A hydrogen line that has a wavelength of 486.1 nm in the laboratory is observed in light from this galaxy.
Calculate the speed at which this galaxy is receding from the Earth.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).