State the postulate of special relativity that is related to the speed of light.

A rocket moving at a relativistic speed passes an observer who is at rest on the ground equidistant from two trees L and R. At the moment that an observer in the rocket is opposite the ground observer, lightning strikes L and R at the same time according to the
ground observer. Light from the strikes reaches the observer in the rocket as well as the observer on the ground.

(i) Explain why, according to the observer in the rocket, light from the two strikes will reach the ground observer at the same time.

(ii) Using your answer to (a) and (b)(i), outline why, according to the rocket observer, tree R was hit by lightning before tree L.

the speed of light in a vacuum is the same for all inertial observers/observers in uniform motion;

(i) ground observer measures a zero proper time interval for the two arrivals;
all other observers measure a time interval of γ×0=0;
hence the arrivals are simultaneous for all observers, including rocket 
Award [1] for statement that “events that are simultaneous for one observer
and occur at the same place are simultaneous for all observers”.

(ii) according to the rocket observer, the ground observer moves towards the signal from tree L and away from the signal from tree R;
since the signals move at the same speed and they arrive at the same time according to the rocket observer;
signal from tree R must have been emitted first