1. 1
   An ambulance drives along a straight road, past a pedestrian standing still at the roadside, with its siren sounding continuously throughout, first approaching and then moving away from the pedestrian.

   (a)

   An ambulance drives past a stationary pedestrian with its siren sounding continuously. As the ambulance moves away from the pedestrian, what happens to the frequency of the siren sound that the pedestrian hears, compared with when the ambulance was stationary?

   [1 mark]

   • AThe observed frequency decreases
   • BThe observed frequency increases
   • CThe observed frequency stays exactly the same
   • DThe sound stops being heard completely

   (b)

   As the ambulance moves away from the pedestrian, what happens to the observed wavelength of the sound waves reaching the pedestrian?

   [1 mark]

   • AThe observed wavelength decreases
   • BThe observed wavelength increases
   • CThe observed wavelength stays exactly the same
   • DThe sound waves become transverse instead of longitudinal

   (c)

   Explain, in terms of wave crests, why the pedestrian hears a lower frequency once the ambulance has passed and is moving away.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A team of astronomers uses a spectrometer attached to a large telescope to measure the wavelengths of light received from many galaxies at different distances from Earth.

   (a)

   Astronomers use a spectrometer to examine light from many distant galaxies. What do they typically observe about the wavelength of light from most distant galaxies, compared with a source at rest?

   [1 mark]

   • ANo change in wavelength at all
   • BA decrease in wavelength (blue-shift)
   • CAn increase in wavelength (red-shift)
   • DThe complete disappearance of all wavelengths

   (b)

   The astronomers find that some galaxies show a greater increase in wavelength than others. How does the amount of red-shift relate to a galaxy's distance from Earth?

   [1 mark]

   • AOnly the very closest galaxies show any red-shift
   • BThe further away the galaxy, the smaller the red-shift, because it is moving away more slowly
   • CRed-shift does not depend on distance at all
   • DThe further away the galaxy, the greater the red-shift, because it is moving away faster

   (c)

   Explain what the astronomers' red-shift observations show about the universe as a whole.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A team of cosmologists uses a sensitive radio telescope to detect cosmic microwave background radiation (CMBR) arriving almost uniformly from every direction in space, and combines this with red-shift data from distant galaxies to test models of how the universe began.

   (a)

   A team of cosmologists uses a radio telescope to detect cosmic microwave background radiation (CMBR) arriving uniformly from all directions in space. Describe what CMBR is and where it is believed to have originated from.

   [3 marks]

   (b)

   Explain how red-shift observations and the existence of CMBR together support the Big Bang model, and evaluate why this model is currently considered the best explanation for the universe's origin.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A public science communicator is preparing a talk for a general audience explaining the evidence that scientists use to support the theory that the universe began with a Big Bang and has been expanding ever since.

   (a)

   A public science communicator is preparing a talk explaining the evidence that the universe is expanding. Explain the observational evidence from red-shift and from cosmic microwave background radiation (CMBR) that supports the idea of an expanding universe and the Big Bang model.

   [6 marks]

   (b)

   Evaluate why the Big Bang theory is currently accepted by scientists as the best explanation for the origin of the universe, and discuss what would be needed for this to change.

   [6 marks]

   Total for question 4: 12 marks

End of questions