Which one of the following statements about the refractive index of a material medium with respect to air is correct?
- It can be either positive or negative.
- It can have zero value.
- It is unity for all materials.
- It is always greater than one.
The refractive index of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as the ratio of the speed of light in vacuum and the speed of light in the medium.
The refractive index of air is approximately 1.
The correct option is D.