So I'm thinking about Einstein's thought experiment in which an observer on a train has a clock that denotes time by bouncing light off a mirror and measuring the time for it to return. The setup is such that for the observer on the train, the light is traveling vertically - strictly perpendicular to the motion of the train. The light's path is just the line up to the mirror, back down to the detector. For an observer on the train platform when the train is moving, the light's path makes an isosceles triangle whose height is the same, and whose base is determined by the velocity of the train (and the height and the speed of light). That is all well established. What I am wondering about is replace the mirror at the top with partially silvered mirror that allows some transmission. The amount of light reflected versus transmitted depends on the angle of incidence. The observer on the train sees the light incident on the mirror at 0 degrees and maximum reflection. The observer on the platform sees the light incident on the mirror at some non-zero angle (the faster the train is moving, the higher the angle of incidence). The observer on the train then expects much higher intensity at his detector than the observer on the platform does. To take it a step further, imagine that on the train the system is slightly angled so that the light from the source is not strictly perpendicular to the floor of the train, and the spot where it reflects back to is actually a hole in the floor of the train. Outside underneath the train, along the tracks, is a detector to measure the intensity of light. What intensity of light is measured at this external detector? If you could swap between closing the hole in the floor and measuring the intensity in the frame of reference of the train with opening the hole in the floor and measuring the intensity in the frame of reference of the platform, would they be the same? See attached image for a crude diagram of the situations.