Chapter 39.4, Problem 39.3QQ

Suppose the observer O′ on the train in Figure 38.6 aims her flashlight at the far wall of the boxcar and turns it on and off, sending a pulse of light toward the far wall. Both O′ and O measure the time interval between when the pulse leaves the flashlight and when it hits the far wall. Which observer measures the proper time interval between these two events? (a) O′ (b) O (c) both observers (d) neither observer

Figure 38.6 (a) A mirror is fixed to a moving vehicle, and a light pulse is sent out by observer O′ at rest in the vehicle. (b) Relative to a stationary observer O standing alongside the vehicle, the mirror and O′ move with a speed v and the light pulse follows a diagonal path. (c) The right triangle for calculating the relationship between ∆t and ∆t_p.