13: Exam Questions

Question 1

Explain how using EM waves you can calculate the radius of the moon’s orbit

Answer 1

(reflect) pulse of (light / radio / microwave) radiation from Moon; measure pulse-echo time;
orbit radius = (pulse-echo time)/2 *c, assume that pulse takes same time out as back

Question 2

When measuring the distance from the earth to an asteroid using a EM pulse, what is a necessary assumption? (4 possible given)

Answer 2

The outward and returning pulses travel for the same time. And the pulse travels at a constant speed (the speed of light)

Question 3

Explain how cosmic background radiation provides evidence for the big bang

Answer 3

any two of the following, [1] each:

1. there is a microwave background radiation;
2. which is red-shifted light from earlier universe;
3. due to expansion of space since big bang;

Question 4

In 2008, a brown dwarf star was discovered orbiting a larger star using the following data obtained
from spectral observations of the larger star. The graph shows the large star’s radial velocity versus
time (here, radial velocity is the component of the star’s velocity towards the Earth). With the use of a diagram, explain why this variation in radial velocity occurs (3)

Answer 4

Reasonable orbit of star and
companion in mutual orbit shown
with Earth shown or direction
towards Earth (1).
Star orbits the centre of
mass[accept ‘common point’] [of
the binary system] (1)
Sensible comment relating radial velocity and position in diagram
(1)[e.g. – in position shown – red shift – longer wavelength; ½ orbit
later – towards Earth so blue shift]

Question 5

The wavelength of light used to obtain this data via Doppler shift was 600 nm.
Calculate the maximum wavelength shift corresponding to the above results.
Velocity of moving galaxy given (1700 m/s)

Answer 5

Δλ/λ = v/c
Substitute values in
3.4 * 10^-12 m

Question 6

Explain why the cosmological Redshift observed in light received from a source increases with the distance of the source from earth
(2)

Answer 6

The cosmological Redshift is due to the stretching of space(time)

(The increased distance means there has been) a greater time of travel (for light)

Question 7

The value of the redshift z = 2.6. By what factor has the universe expanded since the light was emitted?

Answer 7

3.6