Remembering the rule for negative indices: x to the negative 2 is the same as 1 over x squared. This can be written out, in full, as a fraction with four ‘x’s multiplied together as the numera ...

The refractive index of a vacuum is 1.0, and air is a tiny fraction greater than 1.0. The ... over the refractive index (n), thus speed = c/n. Following are common refractive indices.