The time interval At between ticks is 2L (20-1) The interval At is observed by either s or s when the FD clock is at rest respect to that observer Fig 20-4The time interval between ticks is: (20-1) The interval is observed by either S or S’ when the clock is at rest respect to that observer. M F D Fig 20-4 L0 0 t c L t 0 0 2  = 0 t