Are there substances that are allmost invisible in their solid state?

Question

I mean clearer than glass or plexi-glass, that light passes through almost without being bent.

Answer 1

Check out "aerogel". While not literally invisible, when held it looks like a bluish haze, not a solid. Of course the interior (bulk) of a transparent solid like glass is invisible in the sense that you can't see any of the internal material, only the surfaces. But even if the surfaces were invisible, bulk glass would be detectable because its index of refraction is so much greater than air's, whereas aerogel's I.R. is almost exactly the same as that of air.

Answer 2

There exist materials that are very close to what you mean, but only over a part of the spectrum (e.g., X-ray or infra-red).

Potassium Bromide can, with lots of pressure, be made to form such a transparent medium (in infrered, I think); Lithium Fluoride for ultra-violet.

Answer 3

Yes indeed.

Answer 4

Try this site:
http://cat.inist.fr/?aModele=afficheN&cpsidt=16350447