Is there a substance that doesn't reflect or absorb visible light but may reflect light from another spectrum? Is there a theoretical substance that would have these properties?
EDIT:
Sorry I wasn't quite clear with my original question. I've updated it to express more what I was thinking. Would a substance be "invisible" if it didn't reflect or absorb light? Does a real or theoretical substance like that exist? I assume we can see glass and so on because it refracts light and to a certain extent reflects it.
Answer
It's surprisingly difficult to get a material that absorbs almost no light across the whole visible range but one candidate would be black silicon. This has a textured surface created by etching, and the texture means light hitting the surface is multiply reflected sideways before it gets a chance to be reflected back away from the surface. With some absorption at each reflection the multiple reflections mean the overall reflectivity can be a few percent. The material still reflects in the medium to far infra-red because the length scale of the texture is smaller than the wavelength of medium to far infra-red light.
An example of the relectance spectrum is given here:
Response to question v2:
Assuming I understand what you're getting at, the only technology I know of that would fit is metamaterials and in particular metamaterial cloaking. Metamaterial cloaks are effectively waveguides that can bend light around an object so the object in effect neither reflects nor absorbs light.
I should add that we are a long way from actually achieving this in any useful way, and so far what invisible materials do exist work only at microwave wavelengths. However the limitations are essentially technical and the idea would work if we could but fabricate the structures required.
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