You can set up unevesdroppable communication key by sending polarised photons to a receiver. You could do the same using a vampire to block or not at one end, and using a mirror or not to observe at the other end.
Reflective vampires could be used as a power source. Sending a beam of light at them, and absorbing the light that bounced off them, and (by use of a mirror) the light that went through them would give, assuming >50% solar panel efficiency, a greater power output than input. Indeed, stick a reflective vampire in the lasing compartment of a laser minus the laser hole, and you'd get energy out with minimal input energy.
The latter probably means that reflective vampires don't exist; in order to exist without violating the law of conservation of energy they'd have to wear non-light-absorbing black clothing. This would probably be sufficient, provided they avoided reflective surfaces such as items of silver and bodies of water...
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Reflective vampires could be used as a power source. Sending a beam of light at them, and absorbing the light that bounced off them, and (by use of a mirror) the light that went through them would give, assuming >50% solar panel efficiency, a greater power output than input. Indeed, stick a reflective vampire in the lasing compartment of a laser minus the laser hole, and you'd get energy out with minimal input energy.
The latter probably means that reflective vampires don't exist; in order to exist without violating the law of conservation of energy they'd have to wear non-light-absorbing black clothing. This would probably be sufficient, provided they avoided reflective surfaces such as items of silver and bodies of water...