When it is stated that the speed of light is constant, it refers to the speed of light in a vacuum. In a vacuum, light travels at a constant speed of approximately 299,792,458 meters per second (or about 186,282 miles per second) in a specific reference frame. This value is denoted by the symbol "c" and is considered a fundamental constant of nature.
However, you are correct that the speed of light can vary when it travels through different materials or mediums. When light passes through a medium other than a vacuum, such as air, water, or glass, its speed changes. This change in speed is due to the interaction of light with the atoms or molecules in the medium, which causes the light to be absorbed and re-emitted. As a result, the speed of light in a medium is generally slower than its speed in a vacuum.
The speed of light in a given medium is determined by its refractive index, which is a property of the material. The refractive index indicates how much the speed of light is reduced when it enters the medium compared to its speed in a vacuum. Different materials have different refractive indices, leading to variations in the speed of light when it passes through them.
So, when we say the speed of light is constant, we are specifically referring to its speed in a vacuum. In other materials or mediums, the speed of light is different but can be determined by the refractive index of the medium.