I've been attempting to learn special relativity, but I've encountered a stumbling block. I understand that the speed of light is independent of the speed of the source of the light (similar to how sound waves travel at a speed that is independent of the speed of the energy source of those waves). I also understand that the speed of light is invariant across and within all inertial frames of reference. My question deals with the first property. In many thought experiments, e.g. involving moving light clocks (where a photon is bouncing up and down against two horizontally moving mirrors that are separated by a vertical distance), the key motif of the Lorentz transformation (1/√(1-v2/c2)) can be derived by analyzing the geometry of the path of the photon as seen by two observers in different inertial frames, one of which is the same frame as the light clock. To an observer in the same inertial frame as the clock, the photon bounces up and down, but to an observer in a stationary inertial frame (relative to the clock), the photon will take a longer, zig zag path through space. What bugs me is that this thought experiment seems to violate the independence of the speed of light from its source. The very fact that the photon is taking a zig zag path through space, as seen by the stationary observer, suggests that the photon is inheriting the velocity of the clock. The only way that the experiment makes sense to me is if the photon itself is aimed at an angle in anticipation of the mirrors' future position. But if this is the case, the examples we are given never make this explicit. Can anyone deconfuse me?