Laser in front of a wall - mathemathical physics

[voko]

Your earlier definition of $t_L$ was the round-trip duration at a particular angle. In this case, however, you need the times (not durations) of the arrivals back at the origin from two different (but close) angles.

 

[skrat]

Right! Ah, how could I miss that -.-

So, total time should be $t_1=t+t_L$ if $t_L$ is the round trip duration. Similar for $t_2$.

Since the difference $t_2-t_1$ is small, I guess it is the same as if I calculate $dt$ from $t_1$ or from $t_2$.