| Date | May 2013 | Marks available | 3 | Reference code | 13M.3.HL.TZ1.16 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Deduce | Question number | 16 | Adapted from | N/A |
Question
This question is about gravitational red-shift.
An observer is in a space capsule moving at a constant speed in the absence of gravitational fields.
A monochromatic source of gamma rays is fixed to the rear wall of the capsule. An observer at the other end of the capsule measures the frequency of the gamma rays.
The capsule now starts to decelerate.
Deduce the change in the frequency of the gamma rays, as measured by the observer, when the deceleration begins.
Outline, with reference to the principle of equivalence, how the situation in (a) relates to the concept of gravitational red-shift.
Markscheme
the speed of light is constant / the speed of light does not depend on the speed of the source (relative to the observer);
the gamma source appears to be moving towards the observer / OWTTE;
so the frequency will be blue-shifted / have higher frequency / frequency will increase;
principle of equivalence relates acceleration (of frame of reference) to (frame in a) gravitational field / OWTTE;
situation is same (ie blue-shifted) as source falling under gravity towards observer;
Examiners report
Was very poorly done. Candidates had an understanding of photons gaining/losing energy and so changing frequency when travelling through a gravitational field, but had great difficulty understanding the same phenomenon by thinking in terms of acceleration and the Doppler effect. Many candidates tried to do a calculation even though no numerical information was given. It should be noted that the command term deduce means “Reach a conclusion from the information given” and so a calculation is not usually required.
In (b) There were many good statements of the equivalence principle, but most candidates had difficulty relating it to the situation in (a).
