Let the rods be AB and A'B' and have the same rest length of l0. Let the relative velocity of A'B' as seen from AB be v. As seen from AB, thus, the rod A'B' appears to have contracted in length and has a length equal to
. Hence, to someone on AB, it will appear as though the rod A'B' has to travel a distance of 
between the events when its right and left ends meet. Since, the corresponding time interval is 
, we have,
