This question is not directly related with time dilation as much it is with the light clock that is so often used to improvise the concept of time dilation. This clock consists of two mirrors placed parallel to each other, a light blip that bounces between the mirrors. The light blip is periodically intensified to account for the absorption by the photo detector placed on one of the mirrors. Now let’s assume for the sake of clarity that the clock is moving along the x axis and the mirrors are placed parallel to xy plane at some distance z’ from each other. Also let’s assume that the motion is non-accelerated and that it can be represented by vector v Which of these should be correct? 1) Due to the motion of the clock there will be an x component of velocity imparted to the blip and now the sum of two component c in z direction and v in x direction will be equal to c in xz plane. (My guess is that its most unlikely because the motion of source does not affect the speed of light.) 2) There will no horizontal component of light and that will require mirrors to be of infinite length for this arrangement to work as otherwise the blip will continue to shift in –x direction and eventually fall off the clock.