So, your objective is simply to annunciate1
a projectile's presence in the "window", ideally from as close to its breaking the plane
To me, that would be an Arduino misapplication.
I would look at using:
1) an RS Latch (a/k/a
possibly conditioned with a
2) a "one-shot" multivibrator
Still, you need to supply more information.
The projectile has some length and travels at some speed, within tolerances, so you should (need to) come up with a figure for how long the detection event will last, even if ideally so.
Can the IRED emitter/detector operate at that speed?1
- trip an LED, trigger a flash, etc.