Answer: True
Step-by-step explanation:
The speed of light in a material medium depends on its molecular structure, in particular its electromagnetic properties, electrical and magnetic permeability. These properties may have different values for different wavelengths or frequencies of the incident light.
In this way, the speed of light depends on the medium and reaches its maximum value in a vacuum, so that, it will be lower in a material medium . In the vacuum (or in the air) the speed of light is 3 x 10⁸ m / s and in any other medium its value is lower.
To characterize the particularities of the propagation of light in the media, the refractive index is introduced ,
n = c / v
where c is speed of light in a vacuum and v is the speed of light in the medium considered . So the speed of light in a medium is given by the refractive index whose value is always greater than 1.