| Date | November 2014 | Marks available | 3 | Reference code | 14N.3.SL.TZ0.11 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 0 |
| Command term | Explain and State | Question number | 11 | Adapted from | N/A |
Question
This question is about simultaneity.
Daniela is standing in the middle of a train that is moving at a constant velocity relative to Jaime, who is standing on the platform. At the moment the train passes Jaime, two beams of light, X and Y, are emitted simultaneously from a device held by Daniela. Both beams are reflected by mirrors at the end of the train and then return to Daniela.
State and explain the order of arrival of X and Y at the mirrors according to Jaime.
Outline whether the return of X and Y to Daniela are simultaneous according to Jaime.
Markscheme
beam X will reach the mirror first;
the speed of light of each beam is constant for all inertial observers;
the left mirror moves towards the beam X while the right mirror moves away from the beam Y;
the beams returning to Daniela occur at one point in space;
if this is simultaneous to Daniela, the event will also be simultaneous to Jaime;
or
beam X has less to go to the mirror and then longer to Daniela, whilst beam Y has longer to the mirror and less to Daniela;
the sum of the times are the same because Daniela is in the middle so they arrive at the same time;
Examiners report
Usually candidates gave acceptable answers to (a) but forgot that the speed of light must be constant for their statements to be true.
Very few answered (b) well.
