The correct formula to calculate the refractive index of a medium using the speed of light in air (e) and the speed of light in the medium (u) is given by option 2: e/u.

The speed of light in a medium is always less than the speed of light in air or vacuum. When light passes from one medium to another, it changes its speed and direction. The refractive index of a medium is a measure of how much the speed of light changes when it moves from air (or vacuum) to that medium.

The refractive index of a medium is defined as the ratio of the speed of light in air (e) to the speed of light in that medium (u). Since the refractive index is a ratio, it is dimensionless, meaning it has no units.

Therefore, the correct formula to calculate the refractive index of a medium is e/u, as stated in option 2. It is important to note that the refractive index can only be calculated when the speeds of light in air and the medium are known.