Vampires don't have reflections; everybody knows that. But how do they not have reflections? What's the mechanism? And what useful applications are there for that mechanism?

I think it goes like this. When a photon strikes a vampire, it separates into two different types of semiphoton, which I'm going to call an R-photon and an S-photon. The R-photon continues along the original photon's path, as if the vampire hadn't been there; R-photons disappear if they touch a sensor (such as an eye, or a camera), but if they touch a reflective object they reflect off it and turn back into ordinary photons. (Most noticeable with a mirror, but I think it applies to diffuse reflection off ordinary surfaces too.) The S-photon, meanwhile, reflects off the vampire as if it had been an ordinary body; S-photons disappear if they touch a reflective surface (again, ordinary diffuse surfaces as well as mirrors), but things like the photoelectric effect work just fine, so S-photons interact with sensors exactly like normal photons do.

Hence, if you're in the room with a vampire, you see it by means of the S-photons scattering off it; but if you look in the mirror, you don't see the reflections of the S-photons, but only those of the R-photons which went straight through the vampire.

There are a couple of fiddly corner cases: what happens if the first object struck by an R- or an S-photon is another vampire? After drawing a thought-experiment diagram or two on paper, I conclude that R-photons go straight through additional vampires and remain R-photons only, while S-photons bounce off vampires and remain S-photons only. Any other behaviour would sometimes enable you to see a vampire in a mirror, by putting another vampire either between the first one and the light or between the first one and the mirror.

So now we have a sound theoretical basis for the optical behaviour of vampires, what are its applications? Well, distressingly, no application I've been able to think of quite works.

You'd like to be able to use a wide flat vampire as a one-way wall: angle a mirror at the wall and you can see through the vampire as if it wasn't there, but people on the other side don't know you're watching. Except that doesn't work if they have mirrors too, and in a world where people used this as a means of covert surveillance, anyone remotely paranoid would have a small mirror about their person at all times.

A vampire itself would have the useful ability to wear a pair of glasses with one lens mirrored on the inside, and thus be able to see people sneaking up behind it with stakes. Unfortunately, a vampire so equipped would no longer be entirely invisible in mirrors – there'd be a mirrored lens hovering in mid-air. I'm sure that would turn out to be inconvenient.

A superficially impressive military application requires a vampire which itself has a mirror finish. If you take a laser beam and reflect it off a vampire, you split it into an R-laser and an S-laser. The R-laser is useless as far as I can see, but the S-laser has definite possibilities: it has the same effect on a target as ordinary laser weapons, but without the risk of mirror-carrying enemies reflecting it back at you. Except that even that doesn't work, because vampires reflect S-photons; so if the enemy also has a mirror-polished vampire, they can use that to send your beam back at you. Arrgh!

You'd also have to avoid using a laser working in the ultraviolet frequency range, or else your vampire would develop a completely different optical problem…

(Thanks to several people in the pub for helping me discuss this very silly idea :-)