In the proof of time dilation. Below http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm I have a doubt in symetry of time dilation. If there's a big lcd screen digital clock in jasper's verandah(at rest wrt ground). And zoe is moving at speed close to c. Do he see that lcd digital clock's ticks of 1 second is longer than his own clock? Also another question. How could jasper(rest) see the path of light? Do she really see hypotheneus path? Or its just hypothetical that light follow that path? How do we actually got to know light followed hypothetical path? By which instrument?