Travel of Light Back from B to A 


0 = F(t'A)
 A(t'_{A}) = F(t_{B})
 c(t'_{A}  t_{B})
 A(t_{A})  v(t'_{A}
 t_{A}) = A(t_{B}) = A(t_{A}) + v(t_{B}  t_{A}), which we substitute in the above expression to obtain 0 = F(t_{B})  A(t_{B})  (c + v)(t'_{A}  t_{B}). or t'_{A}
 t_{B} = (F(t_{B})
 A(t_{B}))/(c + v) t'_{A}  t_{B} = (B(t_{B})  A(t_{B}))/(c + v) = r_{AB}/(c + v) (see why B(t_{B})  A(t_{B}) = r_{AB} ...). 