Must the refractive index (or index of refraction) of a medium more than one???

Answer 1

In natural circumstances, yes. The reason is that the refraction is caused by a change in the speed of light as it passes through a medium. Light travels fastest in a vacuum, which has an index of refraction of exactly 1. When it passes into a medium and slows down, it will always bend toward the normal, and when it leaves the medium it will always bend away from it.

In fact, you can use the refractive index to directly calculate the speed of light in that medium. If the refractive index for a substance is N, the speed of light through that medium is c/N.

However, recent research in the field of metamaterials has demonstrated a negative index of refraction in very specific manufactured crystalline structures. In these cases however, ordinary refraction is not really what is happening; instead the light is being "channeled" by the fine structure of such materials. And even in these cases, |N| > 1.

Answer 2

Photons of light travel fastest within a pure vacuum. In other materials the photons may be selectively absorbed (giving rise to color) or reflected by interaction with the electrons of the atoms or molecules of the matter. In clear glass that is transparent but magnifies (or a prism), the photons interact with electrons but are not significantly changed except that the path is longer and the trip slower (not faster) depending upon photon wavelength. Think of running a football field with or without (vacuum) a line of tackles waiting for you. Your progress may depend upon whether you are small or large relative to the tackles.

Answer 3

c / v = n (index of refraction)

c is the speed of light in a vacuum
v is the sppeed of light in another medium

Nothing will be greater than c, hence the ratio of c/v will always be equal or greater than 1.

Answer 4

a vacuum will be 1 ,all other media are denser than this so RI >1