Gately, the wraith, and the figurants (pp. 833–834)
It’s not like the light passes through the wraith or anything dramatic—the wraith simply disappears the instant the light hits the heart monitor and reappears the instant it moves away.
Context
The orderly's flashlight as impromptu optics experiment: the narration explicitly rules out the movie-ghost effect (light passing through) in favor of something stranger — the wraith is incompatible with directed light, visible only in the room's ambient gray. The book's great filmmaker, post-mortem, can't hold a shot under illumination.