Yes, the speed of light decreases in media other than vacuum. In a medium, such as air, water, glass, or any other transparent substance, light interacts with the atoms or molecules of the material. These interactions cause a delay in the propagation of light, leading to a slower speed compared to its speed in a vacuum.
The speed of light in a medium is commonly denoted by the symbol "v" and is determined by the refractive index (n) of the material. The relationship between the speed of light in vacuum (c) and the speed of light in a medium (v) is given by the equation:
v = c/n
where "n" represents the refractive index of the medium. The refractive index is a dimensionless quantity that characterizes how much a medium can change the direction and speed of light passing through it.
Since the refractive index of a medium is typically greater than 1, the speed of light in that medium is always less than its speed in a vacuum. For example, in air, the refractive index is very close to 1, so the speed of light in air is slightly less than the speed of light in a vacuum. In denser materials like water or glass, with higher refractive indices, the speed of light is significantly reduced compared to vacuum.
It's worth noting that the exact reduction in speed depends on the specific properties of the medium, such as its composition, density, and optical characteristics.