If ghosts could be seen then its obvious that they are emitting visible light,and light is nothing both electromagnetic fields. which would imply that they are made of physical matter,as everbody knows that electromagnetic fields couple to accelerating charges. What is even more ridiculous is that ghost in popular culture are potrayed in clothes as if clothes have the same kind of after life as the person. If ghosts exist then they are disembodied concious enities. And conciousness is not physical which means it cannot interact with physical matter,unless you take the materialist point of view. I am neither a materialist nor a cartesian dualist but i find both point of views equally compelling. But one thing im convinced about is that ghosts if they exist are not visble to the human eye as they are disembodied conciousness.(assuming that you take the dualist point of view).