The x ′ axis is also tilted with respect to the x axis. To determine the angle of this tilt, we recall that the slope of the world line of a light pulse is always ±1. Fig. 2-3c presents a spacetime diagram from the viewpoint of observer O′. Event P represents the emission of a light pulse at x ′ = 0, ct ′ = − a. The pulse is reflected from a mirror situated a distance a from the light source (event Q), and returns to the light source at x ′ = 0, ct ′ = a (event R).