Consider light from a distant source, sufficiently far away that the wavefront at your instrument is planar. You have a planar sensor array parallel to the inco

Consider light from a distant source, sufficiently far away that the wavefront at your instrument is planar. You have a planar sensor array parallel to the incoming wavefront, so the wavefront arrives at all sensors at the same time. There are different kinds of sensors sensing different aspects of the incoming light. You need to combine them, making sure that you are combining readings that come from the same plane of the wavefront. But one of your sensors takes a little longer than the others to process and produce a reading. You either need to make that sensor start processing earlier than the other sensors, or make the other sensors start processing later, or add some sort of delay between the other sensor outputs and the thing that combines all the sensor outputs. If you fix this by moving that one sensor a little out of your sensor plane, moving it toward the light source, most people, including most scientists, would say that your fix was to make the light arrive at that sensor earlier, because the light in fact arrives at that sensor now before it arrives at the other sensors. Let's say that the sensors process different wavelengths of light. Then another adjustment you might do to get the outputs synced would be to fill the space between where light enters and the sensor plane with a gas that happens to have a low index of refraction for the wavelengths the slow sensor uses and a high index for the wavelengths the other sensors use. That would make the light arrive later at all the sensors than it would have if the gas had not been added, but it will less later at the slow sensor. But again nearly everyone would say that what you fix did was make the light arrive earlier at the slow sensor, because before the fix light arrived at the slow sensor at the same time it arrived at the other sensors, and after the fix it arrives at the slow sensor before it arrives at the other sensors.