What is Dimensional Formula of Refractive Index?

Refractive Index is defined as the relative speed at which light moves through a material with respect to its speed  in vacuum.

Mathematically,
Refractive Index (n) = speed of light in a vacuum (c) /speed of light in a medium (v)

As both the values belong to same  quantity (speed), units gets nullified. So Refractive Index is dimensionless quantity.

Dimensional Formula of Refractive Index= M⁰L⁰T⁰.
Refractive Index is dimensionless quantity.