The refractive index is the ratio of the speed in a vacuum to the speed of light in the given medium. If the refractive index is too low, there will not be a large bending, or deviation, of the light beam from its original path.
The small deviation will make it difficult to carry out the experiment, and is more likely to cause errors. Therefore, larger refractive indexes should be used, in order to ensure that the bending of the light beams is easily traceable.
