The refractive index is a dimensionless number that describes how light propagates through a medium compared to how it moves through a vacuum. This property arises from the way light interacts with different materials. Specifically, the refractive index (n) is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the material (v):
n = c / v
Both the speed of light in a vacuum and in a substance are measured in meters per second (m/s). When you divide these two values, the units of meters per second cancel each other out, leading to a pure number without any units. This characteristic makes the refractive index a dimensionless quantity.
In practical terms, this means that the refractive index provides a straightforward way to compare how much light bends when entering different materials without needing to worry about the units involved. For example, a refractive index of 1.5 means that light travels 1.5 times slower in that medium than it does in a vacuum, but it does not specify the units of speed since they cancel out. This simplicity is part of what makes the refractive index such a useful concept in optics.