Larger than one.
In fact, the index of refraction of a medium is the ratio between the speed of light in vacuum (c) and the speed of light in that medium (v):
[tex]n= \frac{c}{v} [/tex]
But the maximum speed the light can travel at is exactly c, the speed of light in vacuum: so v is always less (or equal, in case of vacuum) than c, and so n is always greater than 1.
