There's an experiment in special relativity that involves a light source and a mirror, both placed in a moving object. It is used to derive the time dilation equation. I'm having serious doubts about it's validity. It implies that light from the source hits the mirror in its exact center, although the mirror has moved a certain distance from the time the light source emits the wave and until it hits the mirror. If, hypothetically, the mirror had moved very fast, then light should have passed by it. But this doesn't happen in the experiment, as if light also had a velocity component in the direction of the train. But it doesn't, since that would imply that the constant speed of light postulate breaks.
Let us dive into the light clock thought experiment,
Special relativity is based on two postulates,
There is no such thing as absolute motion. Phrased another way, all laws of physics should be invariant under changes in inertial frame.
The speed of light is measured to be the same value in all inertial reference frames.
Let's say you are on the train and have light source aimed at a mirror in such a way that the light is traveling perpendicular to the motion of the train. According to postulate 1, there is no way for an observer on the train to tell that they are in motion as the train is an inertial reference frame. Thus the laws of physics should be the same for the observer on the train as it would be for an observer at rest.
From this one can conclude that the light does not miss the mirror and indeed hits it dead center as if the mirror was standing still. If this wasn't the case, then there would be a way to distinguish absolute speed and relative speed, harshly violating the first postulate.
The second postulate leads one (forces one) to the understanding that the amount of time experienced is dependent on motion. Since the speed of light must be constant in all frames of reference and the light travels between two fixed events with different lengths for the moving and stationary reference frames, we conclude that the moving and stationary references frames must have experienced different amounts of time between the two fixed events.