Why quantity refractive index dooesn't have unit

Refractive index is expressed as a ratio of speed of light in vacuum relative to the speed of considered medium. As it is a ratio and the units m/s or km/h cancel out each other. Therefore, the qauntity refractive index does not have any unit. I hope it will be helpful to you. Thank you

luster is not a good characteristic to use by itself because luster is the property of minerals that indicates how much the surface of a mineral reflects light. the luster of a mineral is affected by the brilliance of the light used to observe the mineral surface.

truck y has a greater velocity than truck x.