Yes, the speed of light does decrease when it travels through a medium other than a vacuum. In a vacuum, the speed of light is constant and is approximately 299,792,458 meters per second (denoted as "c").
However, when light passes through materials like air, water, glass, or any other transparent substance, its speed is lower than the speed of light in a vacuum. The speed of light in a medium is denoted as "v" and is typically given by:
v = c / n
where "n" is the refractive index of the medium. The refractive index is a dimensionless quantity that describes how much slower light travels in the medium compared to a vacuum.
Different materials have different refractive indices. For example, air has a refractive index very close to 1, meaning that light slows down slightly when passing through air compared to its speed in a vacuum. Water has a refractive index of around 1.33, and glass can have refractive indices ranging from about 1.5 to 1.7, depending on the type of glass.
The refractive index is related to how much the light is bent or refracted when it passes from one medium to another with a different refractive index. This phenomenon is the basis for various optical effects, such as the bending of light in lenses, prisms, and the formation of rainbows.