有冇物質ge reflective index below1

Yes, there is.

In classical electromagnetic theory, refractive index n can be expressed as
n = square-root[u.e], where u si the permeability of free space, and e is the permittivity of free space.

For most substances, u is more or less equal to 1 in the optical frequency range. Thus, n = square-root[e]. Hence, depending on the values of "permittivity" of substances, it is not impossible that the square-root of the permittivity would yield a result less than one.

Though such result is not common for electromagentic waves in the optical range (or such substance still not disscovered), refractive index less than one can be found in other frequency ranges, e.g. in X-rays. "Plasma" is also a "substance" that exhibits refractive index less than one.

You may see the following web-page:

http://en.wikipedia.org/wiki/Refractive_index

An example is the ionosphere (am ionized layer of air at the upper part of the atmosphere) which has refractive index less than one ( n= 0.8) to certain frequencies of electromagnetic waves. Thus, such frequencies of radio waves are refracted back by the ionosphere. It is because of this phenomenon that trans-continental radio communication is made possible before satellite communication.

Just see the following web-page:

http://answers.yahoo.com/question/index?qid=20080512214054AATbe4C



2009-11-12 08:56:53 補充：
Just a correction. The symbols u and e mentioned above refer to the RELATIVE permeability and RELATIVE permittivity of the substance concerned, not those of free space.