I have like 6 arduino boards just sitting around and I wanna use them, so I think the PIR sensor would work great! Does the PIR sensor form like an "invisible" line? or what? Like If I was able to see the infrared light that it emits, what would it look like?
As mentioned, it does not emit anything, it's Passive. The plastic cover (The multi faceted fresnel lens) is the most common method of focusing the emmited infrared body heat radiation onto the pyrolectric sensors
Here is a good explanation if you want to understand what the sensors "sees"